1 – The definition of Leibniz and Wolff: The “disciples of Ideas” versus the disciples of matter

3The term “idealist” was introduced by Leibniz in 1702 to describe Plato’s doctrine, and was initially used retrospectively and without the consent of the authors involved. In the context it appeared in, the idealist—defined as “the disciple of the philosophy of Ideas” [6]—was one who accorded reality and primacy to the Ideas of a suprasensible world. For Leibniz, they were to be contrasted with the materialists, [7] who only recognized the existence of the matter or mechanisms of nature. Initially, idealists were those philosophers who granted the “world” of Ideas (Eidos) permanence and exteriority relative to our own isolated, individual representations. Evidently, idealism is not opposed to realism in this first semantic configuration. The idealist’s mission is not to deny reality as a whole, and certainly not the existence of a physical world outside ourselves. Instead, it is to attribute “being” to intelligible things, ideas, or idealities. Being is classically defined in these contexts as that which possesses permanence (that which endures or subsists), substantiality (that which remains beneath its accidents), or essentiality (that which transcends my own idiosyncratic representation). As a first, provocative approximation, we could say that Leibniz would happily have included Frege, with his third realm of logico-mathematical idealities, among the idealists. In this first moment, there is no claim that idealists deny the existence of physical reality or the concrete external world. The thought, instead, is that idealists confer the category of being to something other than mere materiality, or to something other than the individual, concrete existences we encounter in our immediate environment. In this way, medieval thinkers’ “essence realism” could just as easily be labeled an “idealism” since, like Plato, they granted a form of existence to ideas (characterized by permanence and self-sufficiency), whereas nominalists argued that the idea of existence, being, or reality could be attributed only to concrete, material individuals—to the horse in front of me, rather than the idea of horseness. Furthermore, this first occurrence of the term does not link idealism with any claims about the subject or its individual representations. The term’s meaning is instead based on its etymology: ideas and ideality. Initial uses of “idealism” therefore refer to those philosophers who claim that ideas and idealities are autonomous in relation to the material, physical world. These ideas form an organized world (a cosmos) or realm independent of our individual or psychological representations. The term corresponds to Plato’s “intelligible world,” and its only opposite is materialism.

4It was Wolff who, a few years later, introduced another way of using “idealism,” one which was not exclusively retrospective, and thereby established it as a philosophical category. In 1719, [8] he included it in a typology that mapped out different doctrines, laying out the conceptual principals on which each had been developed. Every philosopher, Wolff tells us, is either a skeptic or a dogmatist. [9] Any dogmatist is either a dualist (who claims, like Descartes, that there are two substances, extension and mind) or a monist (who claims, like Spinoza, that there is a single substance). Monists are either materialists or idealists. [10] Wolff’s example of materialist monism is Hobbes. Idealist monism can be either “pluralist” or “egoist.” It is pluralist when it, like Berkeley, admits a plurality of minds. The doctrine’s core is simple (at least in terms of Wolff’s categorization): pluralist idealists only recognize the reality (that is, the permanence, and so the “being”) of the world of minds. This plurality of minds, including every rational being, originates in the mind of God. The antonym of idealism here is still materialism. The second branch of idealist monism is exemplified by what Wolff calls the “egoist,” and describes as an exaggerated or hypertrophied idealism, an “extreme point.” It has almost no known representatives except perhaps—as Wolff notes without further detail—“a Parisian Malebranchist” (“quidam Malebranchii parisis”). In the nineteenth century, historians established that this was probably one Jean Brunet, a bookseller who was working in the 1690s. Brunet’s masterpiece had disappeared without a trace, but was supposedly the driving force behind an exotic “sect” called “the egoists.” [11] Note that the egological or solipsistic dimension of idealism seems almost an intrusion: Wolff is clear that this “extreme point” of idealism is more an eccentric outgrowth than a real theoretical possibility. In any case, the relevant meaning of idealism in Wolff’s typology is still the claim that the world of minds is primary and real, in contrast to materialists who recognize only the realm of matter (like Epicurus), or of nature understood according to mechanical laws (like Hobbes). Berkeley—who called himself an “immaterialist,” never an “idealist”—also argued that the important debate was between those who defend the reality of a world of minds and those who recognize no substance except matter. As Berkeley explains, his own “immaterialism” is defined in contrast to “Hobbists” and Epicureans. In his 1739 Metaphysica, Baumgarten adopted Wolff’s taxonomy unchanged, but without mention of “egoism.” Idealism is still contrasted with materialism, but this opposition is far less important, for Baumgarten, than the one shaping contemporary monist-dualist debates (dualists were in the majority, and so set the terms of the dispute). Idealism is mentioned in only two paragraphs (§401 and §438), where the idealist is defined as a monist who argues that mind alone possesses permanent reality; by contrast, materialists (who are also monists) claim matter or extension alone possesses lasting reality.

2 – Diderot’s contribution: The idealist as a member of the “sect of egoists”

5Things might have remained as they were if Diderot had not picked up the term “idealism”—which no philosopher had ever laid claim to, and which had been applied rarely and relatively neutrally to others’ thinking between 1702 and 1740 [12]—and given it a very different definition, which turned out to play a decisive role in the future development of the term’s meaning. He still saw idealism as the opposite of materialism, but argued that its followers denied the reality of the external world, which he defined as the concrete world of physical things. In 1749, Diderot wrote in Letter on the Blind that “[t]he name Idealists is given to those philosophers who are conscious only of their existence and of the sequence of sensations they experience inside themselves, and therefore admit nothing else.” [13] A retrospective illusion might cause us to miss how startling this definition was at the time. Diderot says that idealists recognize the reality of nothing except their own existence: they “admit nothing else.” The notion of ideas or idealities of the intelligible world has completely disappeared. Instead, idealism is simply defined as solipsism, the affirmation of individual consciousness alone—which is, in turn, reduced to a stream of sensations. Even more striking is Diderot’s first example of an idealist to illustrate his bizarre definition: Condillac! Thus redefined by Diderot, idealism resembles a radical empiricism, more closely related to a radical variant of empiricism that Mach would later call phenomenalism. Diderot defined idealism as the claim that nothing outside consciousness exists, either the material or the intelligible world. Diderot neglects the fact that Condillac indeed asserted the existence of external material things, which he argued were the occasional causes of our sensations. And he confuses a merely epistemological thesis with an ontological one. Condillac’s claim, in a passage cited by Diderot, that “it is only our own thought that we perceive,” is a thesis about our own knowledge, and he can still affirm, like Malebranche, that the causes of “what happens in us” lie in an external reality. (Condillac claims that these are material things, Malebranche that it is God.) But Diderot seemingly ignores this, instead transforming this epistemological thesis (that we only know what we perceive in ourselves) into an ontological one (that what is outside us does not exist, has no being). This is the source of an initial confusion—between the ontological (existence) and the epistemological (knowledge)—that will have a long career. Diderot’s second philosopher who falls in the category of idealism is Berkeley. It was Diderot who truly gave form to an image of idealism as synonymous with the denial of the external world, and of Berkeley as its purest representative. Diderot was by no means the first to call Berkeley an idealist, even if Berkeley himself never used the term. Several authors did so after Wolff, but they did not mean that Berkeley denied the existence of the concrete external world. Instead, they meant that he denied the necessity of conceiving of matter as the substrate of bodies, and therefore saw the mind as primary. There is a vast gap between claiming no being or reality exists outside myself, and claiming we must attribute being to God or ideas. This is the gap separating Diderot’s interpretation from that of his predecessors, who still maintained the etymological meaning of the term “idealist,” which linked it to ideas or to the intelligible world. As with Condillac, Diderot’s misrepresentation of Berkeley is total. He read him as an egoist and, a few years later, even claimed that “Malebranche proves that man sees all things in God, and Berkeley that he himself is the only thing that exists.” [14] As Jean Deprun has clearly shown, Diderot is a faiseur d’amalgame (“sower of confusion”), [15] and his whole strategy is to link Berkeley’s name with that of the colorful Parisian sect, which was already mostly fantasy, and which Diderot—who could not pass up such an urban legend—used his literary talents to adorn with some further imaginary features (a legend about the unknown members of a “Parisian” “sect” which had perhaps had only a single member and whose primary treatise explaining its doctrine had gone missing). [16] Diderot radically falsified Berkeley’s thought: as Deprun jokes, Diderot seems to forget that the bishop believed in God! To put the point in more philosophical terms, Diderot seems to forget that Berkeley had only questioned the need to postulate a material substrate underlying bodies, which is certainly not the same thing as denying the existence of all reality beyond our own consciousness. Indeed, Berkeley asserted the reality of sensible bodies, and defended a strong ontological realism. He argued that, while reality depends on ideas, it is not my own mind that is the source of these, but the mind of God: “It is evident that the things I perceive are my own ideas, and that no idea can exist unless it be in a mind […]. [But] I know myself not to be their author.” [17] The author of his ideas, and the origin of their ontological reality, can only be an infinite mind: “Therefore there is an infinite mind or God.” [18] What is most real—what has the most reality—is the mind of God, the source of all reality and the creator of the world.

6So how are we to understand Diderot’s incredible reading of Berkeley? Is it a serious error? Or is it the tongue-in-cheek invention of a literary genius? In our eyes, there is no denying that Diderot is playing a joke (and no denying Diderot’s genius). Elsewhere, Diderot preferred to incarnate the idealist of his dreams (or nightmares) using a fictional character, rather than giving him a well-regarded philosopher’s name. [19] And it seems impossible such a sharp mind would have made such a fundamental error. [20] The only way to understand the incredible, Borgesian fiction Diderot calls “Berkeley” is to reconstruct his underlying philosophical reasoning and, at the same time, the radical rhetorical strategy. The latter is clear: Diderot is a materialist, and he wants to discredit anything that could plausibly challenge materialist monism. His opponent, then, is to be depicted with outrageous, particularly absurd features. Diderot is making use of the rhetorical figure of caricature, which is rarely burdened by concerns about accuracy. The underlying reasoning is similarly easy to reconstruct. For a materialist like Diderot, God does not exist. Nor is there any doubt that mind is the product of matter. Matter is therefore the whole of reality, and reality can be reduced to matter. Denying matter—the “immaterialism” that Berkeley defended—is consequently the same as denying reality as a whole. The idealist denies all reality and, doing so, asserts that objects in the external physical world do not exist. Unlike in the period between 1702 and 1740, the debate is no longer about what being, or permanence, or reality, or primacy should be conferred to—whether to God, or the mind, or ideas, or nature. [21] In Diderot’s work, it instead becomes a debate between those who affirm the existence of an external physical world and those who deny it. Idealism has become a form of solipsism, an acosmism, a phenomenalist empiricism. And it is now to be contrasted with realism, itself reduced to the claim that external physical things exist outside my own representations. [22] Any reference to the term’s etymology, to ideas, or the ideal, or ideality, has disappeared. Any consideration of the possible meanings of the term “being”—does it mean essentiality, substantiality, permanence, or simply the concrete existence of the things in front of me?—has disappeared. And any discussion of whether the notion of truth can be applied to anything other than concrete materiality has faded away. Nobody, except perhaps the phantom bookseller and his ghostly sect, ever positively defined themselves as denying the reality of the external world. Going by such a definition, as Hegel said, “no philosopher has ever been an idealist.” But Diderot’s literary fiction would nonetheless haunt the future of idealism.

3 – Three philosophical moves by Diderot, and the emerging portrait of the idealist as solipsist

7What has this first look backwards yielded? First, no one in the period called themselves idealist; the term could be applied to others, but never to oneself. [23] Second, before Diderot the term was connected to ideas and ideality. The issue was whether being (or more precisely, in the language of the time, reality) could be attributed to things that were not material, singular, and concrete. Diderot was the crucible in which, by some bizarre reaction, we went from realism’s old meaning, illustrated for instance by medieval thinkers’ essence realism, to a new one: the realists were defined solely by their assertion that the external physical world existed. This assumed, in the same move, that philosophers existed who denied such a thing: idealists. Diderot was the first who covered up the etymology of the term “idealism,” associating it purely with the denial of the external world. [24]

8This association was enabled by three implicit conditions, the three theoretical moves of Diderot’s conceptual operation:

91. The first of these, which we have already seen, is an illegitimate move between what we can call an ontological and an epistemological level. The former involves judgments like “X exists” or “X does not exist.” The latter involves judgments about whether X is knowable, whether we can be certain about it, and whether it is true or not. This distinction between levels is necessary: to say that some X is not knowable in some way (through clear and distinct ideas, in the language of the time) does not amount to the positive assertion that X does not exist. Something may exist but still not be the object of a clear and distinct idea. Inversely, a mathematical entity may be the object of a clear and distinct idea, but we may not therefore be immediately required to apply to it the same idea of being or reality we use for concrete things. Being and knowing are not necessarily the same thing. Diderot does not draw this distinction in creating the fiction of the “idealist.”

102. The second condition is what we might call the forced secularization of the enemy—in this case, of Berkeley. Many seventeenth-century systems would descend into the most extravagant solipsism if stripped of their theological foundations. Where would Malebranche be without vision in God, or Descartes without the divine guarantee, or Leibniz without an architect God who regulates pre-established harmony (body/mind and mind/mind parallelism)? Diderot’s strategy, then, is not to claim directly that these systems are false because they require a divine authority that does not exist. Instead, he attributes to his adversaries a doctrine that has had its foundations cut away. So he assigns to Berkeley the astonishing position that he alone exists, even when Berkeley affirms quite clearly that many minds exist, all of them with their foundation and origin in God, who himself exists and is real. This is undoubtedly an error, an amalgame. But this reading is not, in our view, in deliberate bad faith on Diderot’s part. He sincerely thinks that the foundations that rationalist philosophers assert are chimerical. And so he asks, in good faith, what these systems claim when the obvious illusions have been dispelled—or, with his ever-practical approach, what we can do with them. Once we have removed God from the scene, all that remains are our thoughts on the one hand and, on the other, the external, concrete things outside them. We can sum up Diderot’s move neatly as a forced secularization. We have reached the exact moment when God can no longer act as a genuine guarantor. The only things that remain, confronting each other, are thought and the physical world. Some, then, directly define the world as physical—that is, for Diderot, as matter governed by the laws of nature—and argue that it is the origin of all things, against those who instead view thought as the ultimate foundation of all things.

113. The final condition is a corollary of the second. Materialists like Diderot, at least, can no longer conceive of ideas as norms or entities that possess autonomy and, therefore, a sort of reality—like Plato’s intelligible world, or the world of essences, or, more simply, the mathematical world. As the empiricist origin story would now have it, ideas are a series of abstract representations beginning in the senses: this particular blue, and then blue in general, and then the idea of color. Labeling Condillac an “idealist” might have surprised us initially, but it is an important indicator of the move Diderot is making. Up to now, ideas had been correlated with the concept of being and of reality: essences, universals, transcendentals, the intelligible world, the consequences of God. [25] They have been turned, instead, into individual psychological representations. It was a remarkable moment in the history of idealism when, with Diderot, their opponents (and, to repeat, their opponents were precisely those who had dreamed them up in the first place, for no one had ever called themselves an idealist) associated them with empiricism and, more precisely, with the empiricist definition of ideas. [26] We might call this psychologization of ideas a sort of “mentalism” or “ideaism”: the mind deals with ideas, and these in turn are reduced to individual psychological representations serving as intermediaries (“loose floating images,” as Hume would say) between the mind and the physical things referred to. [27] Ideas became synonymous with individual psychological representations, and so could no longer refer (Diderot thought) to universal idealities or essences, like Plato’s intelligible world or Ideas in God. As before, we encounter an equation between reality and matter. Someone who denies matter therefore denies reality. And we now have an equation between ideas and an individual’s own representations. Idealists are therefore those who claim such individual representations are the foundation of the world or of physical reality. These two philosophical operations make the association between idealism and “egoism” possible.

12Bringing together all of Diderot’s clandestine equations, we obtain the following: what is true is what is real (the epistemological is collapsed into the ontological); what is real is, exclusively, matter. So one who raises questions about matter raises questions about the whole of reality. Previously, the concept of reality or existence could also be applied to God or Ideas, but it is now reduced exclusively to the existence of external material things. Ideas themselves are interpreted as the individual’s own representations (empiricist and psychological mentalism) and not as entities capable of esse (essence or permanent reality). And so the conclusion of this threefold operation is clear: idealists are no longer the disciples of Ideas, as Leibniz and Wolff claimed, but of the fantastical sect of the egoists. Naturally, the Encyclopédie endorsed this error, using Berkeley to illustrate its article on “Egoism.”

1 – Rousseau’s equivocation

13Idealism is still present after 1750 in its earlier sense, that is, of being strictly the opposite of materialism. In Germany, however, this opposition was by no means the center of debate, which was shaped instead by the clash between empiricism on the one hand—represented by Locke, who had been broadly taken up by Thomasius, and associated in Hume’s case with skepticism—and, on the other, largely Leibnizian intellectualist metaphysicians like Wolff and Eberhard. This clash, between an empiricism that was to varying degrees skeptical and a rationalism that was to varying degrees metaphysical, was focused on the problem of the relationship between body and mind. The Leibnizians argued for pre-established harmony; the empiricists argued for the psychological origins of thought. While Leibniz’s opponents sometimes called him an “idealist,” this was because he placed value on knowledge by the intellect alone—pure Ideas, rather than sensible representation. In this context, the term “idealism” remained attached to its etymology, without the link that Diderot had drawn with unbridled solipsism.

14This was the situation in German philosophy in the 1750s and 1760s. On the other side of the Rhine, however, Rousseau had been influenced by Diderot, and in the Profession de foi du vicaire savoyard (Profession of Faith of the Savoyard Vicar) introduced an equivocation into the opposition between materialists and idealists. “Materialists” were no longer those who saw matter as the foundation of all things, as had been the term’s philosophical meaning up to that point. That required both monism and the claim that the whole of the world, minds included, was made of matter. Materialists now just claimed that the material world was real. [28] As Deprun remarks, this was by no means an obvious move at the time: “why does the vicar use the word ‘materialist’ to mean a philosopher who asserts the reality of matter?” [29] “It would be more accurate to call them a realist,” wrote Pierre-Maurice Masson in his edition of the Profession de foi. Historically, the source for this is Diderot: materialists are realists who believe in the external world, and non-materialists are those who do not, and who are, consequently, non-realists. This is how Rousseau can declare himself materialism’s greatest enemy (in the first sense: he opposes monism), and in the same text claim that he is indeed a materialist (in the second sense: he believes in the validity of the world that the senses present). This equivocation in the philosophical use of these terms and their opposites is important. It marks an emerging distinction, no longer between empiricists and rationalists (between Thomasius and Wolff) nor between skeptics and metaphysicians (between Hume and Leibniz), but between the great traditional philosophical systems and approaches more willingly reliant on ordinary consciousness, represented by Rousseau’s vicar and also, for instance, by Voltaire’s Candide and its opposition to Leibniz. This second opposition does not overlap with the first: the philosophical issues are not the same and, most importantly, it does not lie on the same conceptual level. This opposition became operative within the philosophical space of the eighteenth century. It was the environment in which the erroneous representation of the idealist would be confirmed, and in which the supposed opposition between realists who believe in the external world and idealists who do not was established definitively. The triumph of this opposition would come with the common sense school in pre-Kantian Germany.

15Our goal here is not to retrace this powerful shift in eighteenth-century philosophy, which began in Germany and spread through pre-Kantian Germany to become the largest philosophical movement in the country by the 1770s. We must limit ourselves to what is relevant for our own topic: the semantic development of the term “idealism,” and the philosophical reasons that its initial meaning (idealists as defenders of Ideas) became equivocal, and eventually fixed with a second meaning (idealists as denying the physical world).

2 – The common sense school’s decisive misunderstanding

16The “common sense” movement aimed to go beyond the opposition between scholastic metaphysicians and skeptical empiricists. These German eclecticists (Spätaufklarung) got their knowledge of English common sense philosophy through Beattie in particular, who was translated before Reid. [30] They set out to criticize both Humean skepticism and Wolffian rationalism. The school’s best known representatives were Johann Georg Sulzer, Johann Georg Heinrich Feder, Christoph Meiners, and Ernst Platner. [31] They were represented by the powerful journal Göttingische Anzeigen, and the group’s main members had all come from the same place: the University of Göttingen, an Anglophile institution which dominated the many translations of English philosophers appearing at the time. [32]

17The movement was characterized by two major theses:

181. The denunciation of metaphysics and its concepts. They considered these chimerical because they were abstract, and useless because they were too far removed from common consciousness and ordinary concerns. The group’s aim was to unseat metaphysics from its status as the premier science, and to substitute an empirical psychology or anthropology in its place. This substitution was crucially important: the nature of what philosophy was to explain had changed decisively. For instance, the eclectics no longer thought that the issue was to explain the structure or essence of the object by means of extension, as Descartes and his successors had. Such explanations, everyone agreed, presuppose an act of understanding by a scientist and do not rely on common sense—which perceives the wax’s secondary qualities perfectly well, and not just its abstract extension. For the eclectics, such inquiries were part of what they referred to, using a label which was already derogatory, as “the abstract metaphysics of the schools” (Schulphilosophie). In their eyes, philosophers instead needed to give an account of psychological consciousness as it is immersed in everyday life. And thus they set out to make empirical psychology, not metaphysics, the basic discipline.

19This shift from metaphysics to empirical psychology provided a fertile environment for the misinterpretation of idealism by Diderot and the French Enlightenment to spread. [33] With it, a lasting split and misunderstanding emerged. The split was between, on the one hand, traditional philosophers who would not speak in terms of psychological consciousness but rather of subjects and objects of knowledge, and on the other hand common sense philosophers whose questions were about our existential, psychological, and ordinary relationship with the world. The divide was between the scholar, aiming to understand the world scientifically and conceiving it solely as an object which can be either true or false, and the psychological subject immersed in this world and seeking to orient themselves within it rather than to know it—to dwell within it, as we would later say. There is a misunderstanding here: common sense philosophers had lost sight of the fact that classical philosophy dealt not with the psychological subject but the scientific one. As Descartes observed, “we were all children before being men”—that is, we are all subjects of a concrete life which must develop before we can undertake philosophical reflection. But Descartes’s claim in his Meditations was not to discuss this ordinary, psychological subject. Far from arguing that each of us, in all circumstances, is a self-transparent cogito, he simply claimed that we need “once at least” in our lives to try becoming such a thing, using an exercise, radical doubt, which he clearly thought had no place in our ordinary life. [34] His purpose was to construct a certain science (“if I wanted to construct something lasting and unshakable in the sciences”), not to illuminate the concrete operations of ordinary psychology—and even less to propose an empirical anthropology. This is a confusion between a doctrine which seeks the conditions for what we would today call “the scientific image” of the world, and another which investigates its “manifest image,” the image which ordinary consciousness produces within its concrete environment. This confusion was the basis for Kant’s fury at a review by two eclectics, Garve and Feder, of the first edition of the Critique. A single exclamation summed it up: “How could they not have seen that the center of the Critique was the question of scientific objectivity, and not of psychological, empirical, ordinary subjectivity?”

202. The common sense school’s fierce opposition to scholastic metaphysical rationalism led it to embrace empiricism and its definition of ideas as individual psychological representations. But they emphatically rejected its harshly skeptical, Humean consequences. They did so by opposing it to “common sense,” defined as any ordinary consciousness’s ability to make legitimate use of a number of cognitive principles (like causality) without asking metaphysical questions about their ontological foundations or epistemological justification. Common sense is an “instinct” that each ordinary person possesses. The issue is to replace clear and distinct ideas—felt to be too intellectual, too abstract—with common sense, defined as instinct: “Good common sense is as sure as instinct in most cases.” [35] Metaphysicians’ hollow and harmful speculations led us astray from this initial, correct path, to which we must now return. [36] To do so, we must recover our original naivety, applying concepts without any desire to prove them by indirect reasons—that is, no longer claiming to justify them except by applying them wisely and in the correct circumstances. Their answer to Hume is simple: there is no question to be raised about the foundations of our belief in causality. The belief itself is proof enough, along with individual instances in which it is applied to different situations. The common sense school saw Berkeley as Hume’s twin, to be refuted the same way: in this case, they appealed to the shared belief in the existence of an external world. Following Reid and Beattie, Berkeley’s metaphysical wranglings were to be confronted with ordinary consciousness, which immediately posits that causally connected bodies exist outside ourselves. [37] From this moment, Diderot’s fiction—of philosophers denying the external world’s existence—had triumphed. Berkeley’s idealism became synonymous with egoism, and it gradually became commonplace even for those who were not sympathetic to British common sense philosophy to associate idealism with egoism and skepticism. This can be seen in Johann Nicolas Tetens’s Philosophische Versuche (1777), [38] or Jacobi’s David Hume über den Glauben (1787), where Kantianism was immediately characterized as “speculative egoism” and “nihilism.” Common sense provided the environment in which this definition of idealism could propagate. It is clearly no coincidence that common sense’s representatives—in the persons of Garve and Feder—would oppose Kant. They understood Kantian idealism on the basis of common sense, contrasting it (as we have already seen in Diderot’s case) with ordinary realism, and no longer with metaphysical materialism. A complete misunderstanding takes place between 1781 and 1783, permanently enshrining the notion that idealists deny the existence of the external physical world, and that realists affirm it.

3 – The philosophical moves which led to the common sense school’s error

21Three philosophical moves gave rise to Diderot’s philosophical fiction: the ontology-epistemology confusion, the forced secularization of Berkeley, and the empiricist theory of ideas. The eclectics reproduced the first and third of these, but not the second, for they were neither materialists nor supporters of the Encyclopedists. And, unlike Diderot, they did not want to defend a strong metaphysical thesis, that everything can be reduced to matter. Instead, they hoped to get away from every sort of metaphysics, whether dualist, or materialist or idealist monism. So the eclectics did not just base their appeal to common sense [39] on the truth of causality or other cognitive principles, but on the existence of God (common sense tells us that God exists) and moral principles (common sense is immediately moral; literature or other improving works offer concrete figures of this ordinary morality). In such a context, the issue is no longer a conceptual opposition between two theses—between matter and mind, between ideas as abstraction and Ideas as archetypical essences, between rational principles as universals and contingent habits. The goal was rather to reject an entire level of explanation, that of epistemological and metaphysical inquiry. Kant saw this clearly: a little later, in 1783, after the Garve-Feder “accusation of idealism,” he realized the true nature of the danger. In the preface to the Prolegomena, he identified the confusion’s source unambiguously: “But fate, ever ill-disposed toward metaphysics, would have it that Hume was understood by no one. One cannot, without feeling a certain pain, behold how utterly and completely his opponents, Reid, Oswald, Beattie, and finally Priestley, missed the point of his problem, and misjudged his hints for improvement—constantly taking for granted just what he doubted, and, conversely, proving with vehemence and, more often than not, with great insolence exactly what it had never entered his mind to doubt—so that everything remained in its old condition, as if nothing had happened.” [40] Kant insisted on this, replying to his imagined common sense critic that “Hume had never put in doubt” such a thing—the common sense school, that is, entered the philosophical debate demanding on the one hand that thinkers like Hume and Berkeley prove something that has never been doubted, and on the other hand appealing, not to epistemological and metaphysical explanation, but to the instinct of common sense. The mixing of levels here is incredible, and having denounced it Kant sighed in disappointment: “I come to suspect that this sort of need of the science perhaps may never have come into [the reviewers’] head.” [41]

22This confusion of conceptual levels lies at the source of the most memorable equivocation, one that substantially influenced understandings of the term “idealism.” The common sense reading of Hume and Berkeley (both now facing the same moralizing scorn) shows a confusion between what we could call an existential and an epistemological skepticism. No philosopher defends existential skepticism, which positively and directly doubts the existence of the external, everyday environment. As Kant pointed out in the passage quoted above, such solipsism or existential skepticism is certainly not what the philosophers in question are arguing about (Hume contra Leibniz, Kant contra Berkeley). Kant does not think philosophers need to offer counter-arguments to existential skepticism, which lies totally outside the philosophical arena. The second position is epistemological skepticism, which simply doubts that there is a correspondence between our rational principles (non-contradiction, sufficient reason, cause and effect, conformity to law) and nature, understood scientifically as a set of regularities, rather than in its everyday sense, the presence of objects physically surrounding me. Epistemological skepticism can be clearly defined as bearing on the ability of our utterances to refer truthfully to the external world, defined in turn as the world that is dealt with by physics. This is precisely the skepticism that Hume defended, and has to do with science. We say the Earth moves even though doing so counters our ordinary perceptions. But how are we to universally guarantee this “scientific image” of the world? Such epistemological skepticism has its defenders (Hume and his followers) and its opponents (Kant, or Eberhardt and contemporary Leibnizians). The common sense school entered the fray by totally falsifying its terms. They create a Janus-faced straw man—half Hume and half Berkeley, half idealist and half existential skeptic—and then earnestly fought it, never realizing that it did not exist either conceptually or philosophically. This Janus character (inherently ambiguous because it dismisses two very different philosophers in the same way) fleshed out Diderot’s appealing caricature. With common sense providing the context, the battle lines were drawn up and the roles distributed. There would now be some philosophers who believed in the existence of our immediate environment—of Earth, moon, and cats—and others who claimed it did not exist, that “[Berkeley] alone exists.” The first would be realists, the second idealists. And common sense will be called on to declare which of the two best reflects its own personal and everyday way of being in the world.

23In summary, the eclectics crystallized the far-fetched representation of idealists as denying the existence of the world into an effective figure. The conditions for this involved two great conceptual operations or philosophical moves beyond those we have already seen with Diderot: 1) Turning ordinary psychological explanations into the content of philosophy, and turning empirical anthropology into the foundation for philosophy as a whole. Philosophers’ statements about the formation of the “scientific image” of the world (clear and distinct ideas, inferences, the non-immediacy of the given, and so on) are taken as claims about its “manifest image” (belief, immediacy of the given). The question of philosophy is no longer that of science, or knowledge, or truth and its definition. This level of reflexivity—of reflection on science, the science of science, the Theory of science—disappears from the philosophical horizon. [42] 2) Contrasting the common sense of ordinary consciousness in everyday life with the epistemological justifications of scientists and metaphysicians. The answer is always the same whatever the question, whether we are trying to determine what is true, good, and so on: common sense provides the right concept in the right situation and, most of the time, applies it.

24These two moves turn the idealist-materialist opposition into one between idealists and realists. Materialists are no longer monists asserting the reality of matter alone, a metaphysical thesis. Instead, they are now mere “realists,” believing in the existence of the external world—an everyday belief. And idealists no longer assert the reality of Ideas, but claim positively that the external world does not exist. Idealists become the opponents of realism, where previously they only opposed one large but specific metaphysical thesis: that matter alone exists. Moreover, the idealist-realist opposition is reduced to a clash between those who believe in the outside world and those who do not. This is a total misunderstanding, and a particularly neat one: there are still no philosophers calling themselves idealists! The attack on idealists precedes their existence; today, we would call it an “alternative fact,” something that is in reality nonexistent. In the environment of common sense and in the wake of the philosophical moves that shaped it, [43] Diderot’s caricature went from a light-hearted sketch to a menace, something to be endlessly flushed out and combated among other philosophers. This profound ambiguity drives the debate with Kant, who uses “idealist” in its etymological sense, as referring to Ideas (non-transcendental idealism) or idealities (transcendental idealism). Garve and Feder instead understand it in Diderot’s sense, which had been definitively crystallized by a surge of common sense philosophy

1 – Idealism in Kant’s pre-critical texts

25Before analyzing Kant’s definition of non-transcendental idealism, we should summarize the three important results from our reconstruction of the trials that the term “idealism” underwent.

261. The first is both obvious and barely reflected in the literature, a sort of purloined letter in the history of idealism: Kant is the first to call himself an “idealist.” All previous instances of the term had attributed it to authors who did not apply it to themselves: Leibniz applied it to Plato; Wolff, Diderot, and the eclectics to Berkeley; and d’Argens, Tetens, and others applied it to Leibniz and Malebranche.

272. Kant’s intervention in the debate over idealism’s historical meaning was provoked externally by the common sense school. We must not yield to the retrospective illusion and suppose that, from the outset, Kant thought that his own position relative to possible older idealists was particularly important. It was only after the Garve-Feder review, [44] which linked him to Berkeley and charged him with “higher idealism,” that Kant became aware of a possible misinterpretation. [45] He responded first in 1783 in the Prolegomena and its important Appendix, which directly addressed this review, and then in 1787 in his changes to the second edition. Strictly speaking, Kant does not defend himself from “the accusation of idealism” in these new texts, as the secondary literature often has it. This is simply because he is the first person in the history of philosophy to call himself an “idealist.” Kant is just responding to the Garve-Feder interpretation of idealism. To summarize, Kant did indeed mention the older form of idealism before these remarks, but did not seem overly concerned about it or (crucially, for our purposes) to understand it as the defenders of Garve and Feder did.

283. Finally, our earlier inquiry reveals a question that Kant’s commentators have left unasked, too often victims of a retrospective illusion that the concept of idealism was widely used, with a long pre-Kantian history. To summarize bluntly, the question is: What is Kant up to? Why does he claim to be an idealist, given that he had denounced the term before the Critique, and given that it was certainly not the center of earlier debates? “Idealist,” after all, was used far less at the time than “dualist,” “skeptic,” or “empiricist.” The answer is simple, so simple that critics have forgotten to give it: for Kant, idealism is always attached to the concept of ideality. This is shown by his understanding of both non-transcendental idealism, which defends Ideas, and transcendental idealism, which defends idealities.

29Having made these remarks, let us examine how Kant understands idealism in his pre-critical texts. This will let us see better the incredible misunderstanding which occurred between the two sides of the debate.

30Pre-critical instances of the term “idealism” are rare, but precise. For Kant, like Leibniz, idealism’s primary meaning has to do with Ideas or idealities (mathematical ones, for instance). Logically, idealists are those who develop theories of Ideas or idealities. We note three important pre-critical instances. In 1755, in A New Elucidation of the First Principles of Metaphysical Cognition, the “idealist” position is plainly attributed to Leibniz and the doctrine of pre-established harmony. The issue is the relationship between the soul and the body. Kant tries to find a solution to reconcile the “materialists,” [46] who make the soul entirely subject to the body and the determinism of the laws of nature (and therefore perishing with nature), and on the other hand Leibniz, who claims that the soul is independent of the material body because their relationship is only indirect, via the pre-established harmony founded in God. These are clearly not philosophers claiming they alone exist, or that the physical world is their creation. There is nothing very original here, then, relative to the pre-Diderot texts we have looked at. In 1766, idealism appeared only once in Dreams of a Spirit-Seer, and almost parenthetically. [47] The reference is to Swedenborg’s claim that a world of immaterial, intelligible beings exists. [48] Idealism is still meant to posit a world of Ideas that reason can reach, Kant tells us, through purely intellectual vision. We therefore have two figures of idealism. [49] One of these is rationalist (Leibniz), the other mystical (or, as Kant calls it, “visionary”). These are certainly different species, but they share a reference to Ideas. Finally, in the Inaugural Dissertation of 1770, Kant says that his new doctrine of phenomena is opposed to idealism, because the doctrine of phenomena “gives rise to very true knowledge.” Does Kant think earlier idealists would have claimed objects do not exist? Not at all. Idealists are those who say that knowledge by the senses is not knowledge which can be true or false; intellectual knowledge alone can be. His example of such an idealist position is the Eleatics, who were the first, in his view, before Plato, to treat mathematics as the paradigm of a purely rational knowledge owing nothing to experience. For Kant, any purely intellectual knowledge or intuition must be rejected. [50] Idealists like the Eleatics, Plato, and Leibniz are simply those who affirm that, since knowledge from the senses is confused, only purely intellectual knowledge can be called true. Such knowledge of the understanding or of reason gives us access to clear and distinct ideas. Mathematics offers an exemplary demonstration of this. Note that Kant unambiguously connects the term “idealist” to its initial, etymological meaning: the disciple of the world of Ideas. Idealists are those who see the world of Ideas as more effective (that is, permanent and essential) than that of those secondary qualities that sensibility offers us.

31In short, in his pre-critical work, Kant clearly understood idealism on the basis of its etymology, referring to the Eleatics, to Plato, and in modern times to Leibniz. Berkeley is not mentioned. Nor is he mentioned in the first edition of the Critique. [51] And Kant does not call himself an idealist in the pre-critical texts, [52] whereas in the Critique he presents himself as the defender of an idealism he calls formal, critical, or transcendental. Given this shift, our question—“what is he up to?”—deserves more attention from the secondary literature, which has taken it as a given that idealism had a long pre-Kantian history, and represented an issue of crucial importance to him. [53]

2 – Non-transcendental idealism in the first edition of the Critique: Every idealist is defined by reference to ideality or Ideas

32References to non-transcendental idealism are relatively rare in the first edition. There are three instances, [54] of which only one is developed at any length, in the Paralogisms. These deal with errors about the soul, not the world—something the reader learned long ago, of course, but which we nonetheless recall here. [55] Descartes is the unambiguous protagonist of the rational psychology of the soul that Kant is attacking. Our interest is strictly in the meaning of the word “idealism,” and the important thing here is that, in the first edition, the idealist is defined “not […] as applying to those who deny the existence of external objects of the senses, but only to those who do not admit that their existence is known through immediate perception, and who therefore conclude that we can never, by way of any possible experience, be completely certain as to their reality.” [56] Kant clearly distinguishes here between an ontological thesis (that objects offered by the senses do not exist) and an epistemological one, which he thinks originated with Descartes: that such objects cannot be objects of knowledge, and therefore objects of certainty, or of clear and distinct ideas. Our sensible experience offers us no necessary knowledge. That does not mean bodies cannot be the objects of any certainty at all. Bodies for Descartes are defined by extension, and this knowledge is guaranteed perfectly. But that just means that this true knowledge (that body is extension) does not arise from our sensible experience, which instead tells us about the color of honey and the smell of flowers. Let us look in closer detail at the fourth paralogism, which was removed from the second edition, but which in the first is when Kant situates himself relative to other possible idealisms. This is crucially important: because we must understand what Kant means by “idealism” before the common sense school’s Janus-faced, Hume-Berkeley character appeared.

33Note, to begin, that Kant applies the term “idealist” to Descartes and not, as in the pre-critical texts, to Leibniz or Plato. This is because of the paralogisms’ precise aim: to offer a critique of the soul and its nature. The fourth paralogism concerns “the ideality of the external relation.” Keeping to the text itself, rather than obscuring it with retrospective or “common sense” readings, that means two things. On the one hand, it means that the idealist—even if they are not transcendental, for which they will in turn be critiqued—claims that the soul stands in relations to objects, and does not claim that objects or external reality do not exist. On the other hand, it means that, for Kant, the idealist is defined on the basis of ideality, something shared by all idealisms whether critical or not—that is, for Kant, whether formal or not. In the fourth paralogism, the type of idealism that Kant situates himself in relation to is that which establishes a distinction between internal sense, which offers truth immediately, and external sense, which provides only secondary qualities, and so does not allow us to access the primary quality, extension, which only understanding can know. This is an epistemological skepticism about the value of the testimony of our external senses: the question is about knowledge, and the claim is that the senses do not offer us trustworthy knowledge on which to build “something lasting and unshakable in the sciences.” Such Cartesian idealism assumes a sort of infallibility of reference for objects of internal sense, but not for those of external sense. I cannot be mistaken about the fact that I have such-and-such a sensation, perception, or thought. But I can be mistaken about the nature of the objects of these sensations or thoughts. Similarly, I can have illusions about an external object (some object in space—the stick bent in the water, for instance), but I cannot have illusions about the fact that I have this perception (about my internal sense). This thesis establishes a strict hierarchy between internal and external sense, and is what Kant calls “empirical idealism.” It is “empirical” because it concerns the experience offered by sight, touch, hearing, smell, and taste. But why “idealism”? Why not “epistemological skepticism,” since we could seemingly attribute this position to skeptical empiricism too? One answer is that the context, of the paralogisms and the rational doctrine of the soul, calls for a discussion of Descartes rather than Hume, who is refuted elsewhere. This is true, but it is not enough. Kant talks about “idealism” because, for Descartes, our knowledge of the nature of external objects—something Kant never believed that the earlier thinker denied the existence of—comes through the understanding, not the senses. Knowledge of the reality of the object as a cause of sensations is thought and deduced, rather than experienced, “added by thought and obtained as the conclusion of an act of reasoning.” [57] A genuine skeptic like Hume would have claimed that we could have no valid knowledge. The empirical idealist instead says that we can have valid knowledge, but that it is not given by the immediate experience of our senses (by the quality of the piece of wax) but by the understanding, which apprehends extension. For Kant, there is a clear dividing line between skeptics and idealists: for skeptics, true knowledge is not possible; for idealists, it is. Once this crucial point is established, a division between two sorts of idealism can now be made, based on the status that they accord to experience, and whether they think that knowledge of it is possible, as Kant does, or not. The second position is defended by non-transcendental idealists, who claim that knowledge arises by detaching itself from the experience of our senses. We are far from the conceptual confusion of the common sense school, who consider Hume and Berkeley, skepticism and idealism, as two sides of the same coin, the negation of the existence of the external world.

34The crux of Kant’s argument against Cartesian idealism is the rejection of the idea that a hierarchical relationship exists between an internal sense offering certain knowledge, and an external sense whose objects are accessible only through dubious inferences, as Hume argued, or through uniquely intellectual ideas, as with Descartes’ extension when supplemented by the divine guarantee. Kant shows that external sense can offer true knowledge and, consequently, that he himself is an “empirical realist.” But his refutation is less interesting for us than the following three points. [58]

351. His portrait of a non-transcendental idealism. Kant defines idealism based on the concept of ideality, and is perhaps even unaware, in the first edition, that a conception of idealism exists that identifies it with existential egoism.

362. His ability to synthesize two terms, “realist” and “idealist,” in a way he thinks is coherent. Both he and Descartes are realists and idealists at the same time. Descartes is an empirical idealist but a transcendental realist, and Kant is a transcendental idealist but an empirical realist. Much has been written about this realist-idealist pairing, which critics have universally viewed as invented or as merely formal, because it unites two such violently opposed terms. For Kant, however, realism and idealism are not absolute opposites and, given the history we have traced out, there is no reason for them to be. The terms are absolute opposites only if we start from Diderot’s opposition, taken up by the common sense school, between those who assert the existence of the external physical world and those who deny it. For Kant, one can be “idealist” from one (empirical or transcendental) point of view, and realist from another. He had no reason to think that the two terms were on their way to becoming absolute opposites, by way of the common sense school and, a few years later, Jacobi.

373. In the pre-critical texts, idealism is not directly connected with what we could call a philosophy of the subject. The theme of subjectivity would be wholly out of place here: the idealism of Leibniz, Plato, and the Eleatics is defined instead on the basis of Ideas, and in this case on the rejection of knowledge through sensible experience (empirical idealism). The same is true in the Paralogisms, despite Descartes’ inclusion as an idealist, and the fact that the issues are errors about the soul. For Kant, idealists have theories of ideas or idealities, and also claim that valid knowledge is possible—which is the difference between idealists and skeptics.

38This gives us a very precise definition of idealism, one confirmed by Kant’s immediate reaction to the Garve-Feder review.

3 – The 1783 response and the three defining traits of every idealism

39In the Appendix to the Prolegomena, Kant contrasted his definition with the one used by Garve and Feder, which he seemed only now to be aware of, and thought made no sense. Kant’s definition remained true to Leibniz’s, and to the word’s etymology: “The thesis of all genuine idealists, from the Eleatic School up to Bishop Berkeley, is contained in this formula: ‘All cognition through the senses and experience is nothing but sheer illusion, and there is truth only in the ideas of pure understanding and reason.’” [59] For earlier idealists, the things delivered by the senses or sensible experience (and only by them) cannot be either true or false. The second part of the sentence emphasizes that what can be thought of in terms of truth, for idealists, are “the ideas of pure understanding and reason.” Without too much inaccuracy, this can apply equally well to Plato, Descartes, Leibniz, and Berkeley. [60] The point is not just that Kant is a better historian of philosophy than Diderot, who thought Berkeley admitted no existence beyond his own. It also emphasizes a subtle but decisive point about this history of idealism. The reality of sensible, concrete, external bodies is not an issue here, and Kant offers no answers on this point. He simply does not bring it up as a possibility. He does not consider the question because, unlike Diderot and the common sense school, non-transcendental idealism is defined by the concept of the Idea, and not by an existential skepticism based on egoism or radical subjectivism. We can get to the heart of this point through Kant’s immediate distinction between his own transcendental idealism and transcendent idealism. He writes: “The principle that governs and determines my idealism throughout is, on the contrary: ‘All cognition of things out of mere pure understanding or pure reason is nothing but sheer illusion, and there is truth only in experience.’” [61] This is Kant’s famous empirical realism, which he had already developed in the Paralogisms. He sees the difference as simple: for non-transcendental idealists, the world of sensible experience is not open to valid judgment. Truth is to be found elsewhere, in Ideas. This explains why they are called “idealists”—with reference to Ideas—and not skeptics like Hume. For Kant, sensible experience is not just open to knowledge, but is precisely that which valid knowledge can be about. Responding to the Garve-Feder review, he showed why “especially Berkeley” defended what Kant called a “visionary idealism,” using an adjective he had applied to Swedenborg’s idealism. Berkeley is visionary and dogmatic because he posits an exclusively intellectual knowledge: “visionary idealism […] (as was already to be seen with Plato) always inferred, from our cognitions a priori (even those of geometry), to another sort of intuition (namely, intellectual) than that of the senses.” [62] Schwärmerei is both the limit and the consequence of such dogmatic idealism, and its foundation is intellectual intuition.

40We can thus draw a portrait of idealism as Kant understands it using three features that precisely describe the concept and its use.

411. A theory is idealist that engages at some point with Ideas or idealities. Beyond their specific difference—whether they are transcendental, empirical, material, problematic, dogmatic, and so on—all idealisms share these terms.

422. Idealism asserts that true knowledge is possible, as opposed to Humean radical skepticism, or what we would today call relativism.

433. (a) Idealism is not the absolute opposite of realism. Both can be attributed to the same doctrine in different ways. (b) Idealism never depends on or derives from a philosophy of subjectivity: because Plato, the Eleatics, etc., are all idealists. Once again, shared terms (ideas and idealities) are the most important feature for determining if a philosopher should be grouped with the idealists.

44The misunderstanding that separates this rigorous definition and subjectivist, skeptical, existential solipsist depicted by the common sense school is, quite clearly, total. The latter depiction has no model and no referent: it is made up, an improbable Janus-faced character in which Hume and Berkeley are run together. We can combine their names and baptize the character as “Huber,” marking it once and for all as a mythical creature. Our aim here has been to explain the moves involved in composing its portrait.

45The story of this mythical idealist—the mysterious member of the sect of the egotists, who became the eclectics’ Huber—obviously does not stop here. Nor does the story of idealism proper, that idealism vouched for in the texts of those who claimed the label for themselves! The three defining features we have used to describe the triangle of real idealism are to be found in every philosopher after Kant who declared themselves an idealist: in Hegel, in Husserl, and so on. [63] And the mythical idealist, too, continues his journey, to Moore and his “refutation of idealism,” and to more recent “realists.” His is a parallel path, one that never intersects the historical figure of the genuine idealist, but which periodically reemerges, like a rabbit out of a hat—a trick whose workings we have tried here to expose: the collapsing of the ontological and epistemological levels, the reduction of ideas to individual psychological input, the rejection of metaphysical and epistemological explanations in favor of descriptions of ordinary consciousness, the confusion of “manifest” and “scientific” image of the world, and the appeal to common sense.

46We were seeking an “invariant,” a false but persistent figure, and we have found two. The first is the “mythical idealist”—a character without a historical referent, and therefore empty. The second is the real figure, always structured across its different instances (whether Kantian, Hegelian, or Husserlian) by the three traits that we have described. More importantly, we have traced a split that goes far beyond the one, foreign to Kant, between idealist and realist. On one side of this split is a philosophy with the question of truth at its core, conceiving of the world theoretically, as an object of knowledge, and conceiving of philosophy itself as a theory of knowledge (or of bodies of knowledge in general, whether theoretical, practical, historical, and so on). On the other is a philosophy with the question of existence at its core, and which approaches this by asking how ordinary consciousness can or should orient itself in the world. This split, we found, was the source of the misunderstanding between Kant on the one hand and, on the other, the common sense school, which approached philosophical questions existentially, rejecting both metaphysics and that particular level of philosophical reflexivity that presents itself as a reflection on knowledge and science. This split goes far beyond the clash between idealists and non-idealists. Even if the term “theory of science” remains associated with a single great idealist, many others lay claim to it beyond idealism: Bourdieu, for instance, who called one of his books Science of Science and Reflexivity, or Cavaillès and his Théorie de la science (Theory of Science). Partisans of “common sense” reject this level of reflexivity, this dimension of the “theory of knowledge,” or they simply fail to notice it (in Kant’s view), whereas this level provides the framework for the projects of the classical philosophers: for Descartes, Leibniz, and as we have seen, for Hume himself. Our aim here has been to trace the emergence of this split, which lies at the origin of the misunderstanding that dominated the battle between two idealisms: one dreamed up by its enemies, lacking any historical referent at all, and the other claimed by Kant, the first ever to do so. This split shaped the opposition (as deep as it is invented) between realism and idealism. It continues to haunt debates in philosophy today, and even determines in part the form that they take.