1There is a widespread belief that Berkeley and Spinoza have little in common, because Berkeley shares none of materialistic, pantheistic, or deterministic views he attributes to Spinoza. Indeed, he refers to Spinoza as a declared enemy of religion (NB 824) and “the great leader of our modern infidels” (Alc. VII. 26), and he identifies Spinoza –along with Vanini, Hobbes, Leibniz, Bayle, and Anthony Collins– as a closet atheist and fatalist because of his identification of the universe with God (Alc. IV. 16). 
2However, Berkeley also admits that, like Spinoza, he endorses St. Paul’s doctrine that “we live and move and have our being” in God (NB 827; PHK 66, 149). Furthermore, for Berkeley, nature is not “some being distinct from God,” for God is the principle by which all things are perceived according to certain laws (PHK 150). However, as Berkeley’s early critics remark, this doctrine seems to link him to Malebranche (and thus indirectly to Spinoza), in that it implies that our ideas are expressions of God’s essence. 
3Berkeley objects to being linked to Malebranche, noting that the passivity of our ideas is inconsistent with God’s active nature (DHP 213-214). But it is difficult not to detect Spinozistic themes in his Siris pronouncements that “God alone exists” (S 344), and “so long as mind or intellect is understood to preside over, govern, and conduct the whole frame of things,” the universe can be conceived to be God, and creatures can be conceived to be “partial manifestations of the divine essence” (S 326). Berkeley insists that there is nothing atheistic about considering God together with nature “as making one whole, or all things together as making one universe” (S 300), for such a view acknowledges the immanent presence of the divine mind in the differentiation and harmony of all things without equating that mind with those things.
This, of course, is the same point that Spinoza makes in distinguishing between God as the free, though immanent cause of the universe (natura naturans) and the determined and determinate universe he creates (natura naturata, S 299). So it is only by failing to note the nuances of this distinction that Berkeley can develop his caricature of Spinoza. That is why my focus is less on what Berkeley says about “Spinoza” and more on how their actual doctrines distinguish them from many of their contemporaries.
I – Bergson and Peirce on the Spinoza-Berkeley Connection
4To discern the features of the distinctive insights of Spinoza and Berkeley, I turn to two thinkers, Peirce and Bergson, who single Spinoza and Berkeley out as kindred spirits. For Peirce, the two are intriguing because of their emphasis on the intrinsic relatedness of ontological categories.  For Bergson, the two are notable because of their clarification of what things are. 
5Bergson focuses on how, for Spinoza, there can be no more than one substance, because thinking of more than one substance would require that we invoke a principle of differentiation and relation (E I P5).  Because that principle itself cannot have a determinate identity, it cannot be anything other than the activity of differentiation and association in terms of which intelligibility is recognized.  For Bergson, the identification of Spinozistic substance as some thing cannot therefore simply be a given, even if it is conceived only in and through itself, for there is no principle of individuation on which “its” positing can be based. Instead, its intelligibility consists in being the immanent principle of potential relatedness of all things. As such, it already contains within itself the differentiation of modes that Descartes is hard pressed to explain and justify.
6The vexing questions of how the variety of finite minds or ideas follows from infinite thought, or how finite bodies follow from infinite extension, are thus solved, for Bergson, by recognizing how, like substance itself, the attributes of substance (thought, extension) are not things at all, but rather ways of conceiving (i.e., “realizing”) objects as minds, ideas, and bodies. Accordingly, to say that thought or extension is infinite means that it is the undetermined principle (or “substance”) by which a mind, idea, or body is identified and differentiated in relation to all other minds, ideas, or bodies. Prior to its realization as a determinate (and thus determined) mind, idea, or body, everything is merely a “reservoir of possibilities” (CM 137). Once something is realized or cognized as this or that “idea,” its identity is established and its relations are determined. For Bergson, “that is why Berkeley prefers to call bodies ideas rather than things,” for unlike an idea, a thing (such as the principle of determination itself) can be indeterminate (CM 137). By having an idea of something, a particular mind identifies a particular thing as “a completely realized existence, whose being is indistinguishable from its seeming” (CM 137). Not surprisingly, then, Berkeley admits that a spirit is a thing by whose activity a body is perceived as this or that idea.
7By drawing Berkeley and Spinoza together on this point, Bergson shows that an act of perceiving (i.e., realizing) a body also determines a mind as the specific mind that has this specific idea. To the extent that individuals are perceived as intrinsically related, they are understood as elements in a divine harmony and thus recognized in terms of their ultimate principle, God.
8Peirce, in turn, associates Spinoza and Berkeley because they anticipate pragmatism’s penchant for describing bodies in terms of their effects, not their causes. Peirce argues that this is significant for understanding Berkeley, because if the cause of some thing’s existence is not another thing –which is what Berkeley insists on in appealing to mind or will as the cause of ideas (DM41-42)– then the cause of our determinate ideas is forever beyond our cognitions. 
9In Peirce’s terms, the pure givenness of anything (“firstness”) can thus be understood only by thinking of it as something determinate (“secondness,” Spinoza’s mode or Berkeley’s idea).  To think this or that thing as meaningful, we further have to recognize it in terms of its relations to other things (“thirdness”). But why things are related in specific ways is itself explicable only because (1) there is (inexplicably) some fact that is cognized as (2) this or that thing. Apart from one another, the indeterminacy of firstness and the haecceity of secondness cannot be explained because they are themselves the principles of explanation (CP II: 475).
10As Spinoza might put it, no thing justifies the pure, indeterminate activity of substance; nor is there a specific reason for the individuation of modes in relations governed by laws of nature. Together, however, these three ontological principles provide a framework in which an explanation of existence is possible, because they provide a framework for explanation itself. In a similar way, Berkeley’s account of the intelligibility of objects in terms of mind provides the conditions of explanation by identifying the presuppositions for claims about existence.
11Peirce acknowledges, however, that Spinoza and Berkeley differ in their descriptions of how individuals are differentiated. Spinoza emphasizes the modal determination (secondness and thirdness) of things, whereas Berkeley emphasizes God’s immediacy in the identification of ideas in relation to one another (firstness and thirdness).  But for Peirce both thinkers differ from their contem-poraries in that they deny that the causes of things are things themselves.
In regard to Spinoza in particular, this means that determinate minds, ideas, and bodies cannot be explained as different expressions of some determinate substance, for that would suggest that substance is some determinate thing from which other determinate things follow. But infinite substance cannot be the differentiating cause of finite substances –as it is for Descartes regarding finite minds, Malebranche regarding individual ideas, and Hobbes regarding individual bodies– for if substance were understood as a thing [res] rather than as the principle by which things are identified and differentiated, it would have to be determined. So Spinoza concludes, “Things [res] could have been produced by God in no other way, and in no other order than they have been produced” (E I P33); and “there is absolutely nothing in things [rebus] on account of which they can be called contingent” (E I P33S1) –not because the principle of determination is itself determined, but because things cannot be things without being determinate.  To assume that the principle of determination is determined would be to assume that substance (i.e., God) has a determinate identity, and that would require a further principle of determination.
On this point Peirce draws on the same Berkeleian insight as Bergson: the principle by which things come to be is not itself an it. As Bergson recognizes, even to call it a thing risks confusing it with an idea, which is the effect of such activity (cf. NB 658-659, PHK 39). No doubt Berkeley is willing to say that we can know the “substance of a body” (NB 701) or “substance taken in the vulgar sense” as the combination of sensible qualities (PHK 37; also NB 724, PHK82, DHP 237-238). But he prefers to say of the “substance of a spirit” that “it acts, causes, wills, operates, or if you please (to avoid the quibble that may be made on the word it), to act, cause, will, operate; its substance is not knowable, not being an idea” (NB 829).  In this way, “there is not any other substance than spirit” (PHK 7), because substance is properly nothing other than the will that there be things differentiated in determinate relations.
This way of speaking about substance avoids Descartes’contradictory description of minds and bodies as contingent (i.e., depen-dent) things that do not depend on anything else for their existence.  It also avoids the skeptical challenges occasioned by Locke’s claim that mental operations and sensible qualities exist only in unknown thinking and material substances.  These problems dissolve when we no longer think of substance as a thing but rather as the activity by which things come to be –for Spinoza, as embodied, and for Berkeley, as willed.
II – Will as the defining feature of extended substance
12To see how Berkeley draws on Spinoza in shifting the understanding of substance from a thing to the activity that accounts for the existence of things, it is important to note how his most direct engagements with Spinoza’s texts occur in his Notebooks precisely where he develops his most explicit views on substance, will, and freedom.  It is no accident that among Berkeley’s comments on Spinoza’s works (NB 826, 827, 831) we find the NB 829 passage I cited above on how the substance of a spirit is not some thing or “it” but rather a verb: to act, to cause, to will. That defining characterization of substance is set up in NB 827 and 828 where Berkeley writes:
“Spinosa (vid: Pref. Opera Posthuma) will have God to be omnium rerum causa immanens, & to countenance this produces that of St. Paul, in him we live, etc. Now this of St. Paul may be explained by my doctrine as well as Spinosa’s or Locke’s or Hobbes’or Raphson’s, etc.”
“The Will is purus actus or rather pure Spirit, not imaginable, not sensible, not intelligible; in no wise the object of the understanding, no wise perceivable”.
15These notes signal how Berkeley situates his discussion of substance in the context of Spinoza’s comment to Oldenburg that “God is the immanent cause, as they say, of all things, and not the transitive cause.”  No doubt, any cause can be distinguished from its effects. But God does not exist apart from his creation, for that would mean that he exists apart from his activity, which would make God an abstraction, a thing that just happens to think (perceive, will) in certain ways. Like any mind, though, God’s activity is essential to his existence, so his activity cannot be separated from his existence. As Berkeley observes, “whoever shall go about to divide in his thoughts, or abstract the existence of a spirit from its cognition, will, I believe, find it no easy task… To frame a notion of a spirit’s existence abstracted from its thinking, this seems to me impossible” (PHK 98, MS addition to PHK 98).  God in particular cannot be considered an it, because the will that there be things cannot be a thing. In this sense, the God in whom “we live, and move, and have our being” is the immanent and not transitive cause of all things.  To think (as Hobbes does) that the causal principle of ideas (viz., will) is itself an idea is to confuse a cause with its effects (NB 806, 808). And to think (as Descartes and Locke do) that mind or spirit is distinct from its activity “as a thing from its modus or manner” (NB 798; also NB 795-796) is to appeal to a blatant abstraction.
16For Berkeley, then, God is not identical with the differentiated things in nature, for the act or will whereby things are differentiated is not itself differentiated. As Berkeley puts it, “Tis One Will, one Act distinguished by the effects. This will, this Act is the Spirit, operative Principle, Soul, etc.” (NB 788). But because differentiation is an effect of will, and because mind or spirit is itself defined as will, the differentiation of minds or spirits can be discerned only in terms of their effects. This means that minds are differentiated in terms of their having different ideas.
17Again, we see a similarity between Berkeley and Spinoza, here specifically regarding Spinoza’s claim of the symmetry of minds and bodies. That symmetry, though, applies only to how things are related to one another, not to how they are related to God. For even though a finite mind’s imaginative or willful activity simula-tes God’s ability to create things out of nothing (NB 830), it does not explain the fact of such activity in the first place. That is why Berkeley warns himself: “I must not give the soul or mind the scholastic name pure act but rather pure spirit or active being” (NB 870), for the act by which a particular spirit or being is produced is itself not identifiable other than in the derivative terms of the effects of pure act.
18Accordingly, Berkeley is unwilling to characterize individual acts of will (i.e., volitions) or even the activity of willing itself as things. Nor is he willing to say that volition or will even “exists,” for to do so would imply that the will is some thing (NB 792). He decides, then, to avoid speaking about “the will” itself, for appealing to such an abstraction ignores how the differentiation and association of things is not the result of volition but, in fact, is volition. “Tis folly,” he writes, “to define volition an act of the mind ordering, for neither act nor ordering can themselves be understood without Volition” (NB 635). Or as he explains more fully:
“The grand cause of perplexity & darkness in treating of the Will, is that we imagine it to be an object of thought (to speak with the vulgar); we think we may perceive, contemplate & view it like any of our other ideas, whereas in truth ’tis no idea… If you say the will or rather a volition is something, I answer there is an homonymy in the word thing when apply’d to ideas & volitions & understanding & will”.
20We cannot think of “the will” as a thing, because we are able to think things only as products of will. In this way, to think is to will, for the act of discerning an object consists in its becoming an object of thought (i.e., an “idea”). Therefore, “there can be no perception, no idea without will” (NB 833; also NB 821), because a perception or idea is unintelligible apart from the affective relations that define it.
21Nonetheless, as Spinoza points out in a letter that Berkeley cites (NB 844), “the will is only a being of reason and ought not in any way be called a cause of this or that volition, in that particular volitions cannot be called free because they require a cause to exist.”  What individuates a volition is the fact that it is the will that there be a particular object. But such particularization cannot be intended by will as such, because the identity of an object as this or that idea is established only as the differentiation and association of objects. God and finite minds are different, then, only in terms of the extent, complexity, and harmony of their objects, for “the Spirit, the active thing, that which is soul and God, is the will alone” (NB 712). Soul and God are thus essentially the same; only the products of their activity distinguish them.
22In thinking a thing, I am therefore expressing the will that there be an object for a consciousness that reflexively is identified as my will (NB 744). But the will that there be objects for my consciousness cannot be said originally to be “my” will, because I am defined only by the activity of those ideas being differentiated and associated in and as a determinate order.  This is the sense in which soul and God are one. That there be a particular order of ideas that are objects of consciousness for me is for all eternity the will that I be: that is why in existing or having any idea, “I am eternally, constantly willing” that there be a “present state” I can identify as my own (NB 791). This “acquiescence” (as Berkeley calls it) is not an affirmation that I could choose not to make, for the affirmation of this sequence of ideas is my existence. As deterministic as this sounds –and Berkeley acknowledges that fact, noting “you tell me according to my doctrine a man is not free” (NB 631)– it is, in fact, an indication of how thinking a world that is one’s own is a determinate expression of the divine substance.
23Furthermore, to say I am eternally willing means that for all eternity there is a will, such that “things, with regard to us, may properly be said to begin their existence, or be created, when God decreed they should become perceptible to intelligent creatures, in that order and manner which he then established, and we now call the laws of nature” (DHP 253).  As an expression of God’s eternal decree that there be a determinate sequence of perceptions that identifies things in law-governed relations, I embody (albeit in a limited way) God’s knowledge of all things. For all eternity God has willed that I know exactly what I know, and in this sense my differentiating and associating things in a determinate way is identical to what God has intended (and thus known) for all eternity. In this sense, “the soul taken for the will is immortal” (NB 814). My ideas, however, are not the same as God’s ideas, because my ideas are passive and limited and do not indicate how all things are related harmoniously with one another.
24These last points, Berkeley argues, distinguish his views from those of Malebranche, in that Malebranche thinks that we become one with God by knowing things in terms of his ideas (NB 230, DHP 213-214). For Berkeley, however, our union with God is not based on sharing in God’s ideas, for ideas are the effects of will by which minds are distinguished from one another. Instead, we are one with God because we are eternal expressions of the will that there be specific differentiations and associations of things. Specific identifications of ideas-in-relation thus define both our existence and the existence of things in history, and apart from that activity there is no distinction of one moment from another or one mind from another.
25As Spinoza indicates in another letter that Berkeley cites (NB 831), this means that every thing and its determinate characteristics (i.e., “affections”) is an “eternal truth,” in that the existence of all minds and bodies is determined for all eternity.  However, no specific determination of things is implicit in substance; for even when substance is considered as extended, it does not have any characteristics of matter or bodies (e.g., divisibility, measurability). According to Spinoza, that is precisely what is wrong with Descartes’ definition of matter as extension, and it is why the variety of things cannot be deduced a priori.  Instead, extension cannot be thought apart from how substance is objectively realized as content. When Spinoza writes to Oldenburg, then, that “extension is conceived through itself and in itself,” he is pointing out that substance cannot be conceived other than in terms of its attributes, and that attributes cannot be conceived other than in terms of substance.  This way of speaking about extension is unlike that of Descartes or Locke, because for them extension is intelligible apart from substance (and apart from thought as well). Berkeley’s explicit contradiction of Spinoza’s comment to Oldenburg (NB 844) is thus not a rejection of Spinoza’s doctrine but of how “extension” is understood by others.
As evidence for this, we need look only at Berkeley’s previous entry (NB 843), where he alludes to Spinoza’s distinction between idea and ideatum (the object or content of the idea). For Berkeley, an idea is a passive object of perception, so Spinoza’s distinction between idea and ideatum (E II D4, D4exp) strikes Berkeley simply as a distinction between a real sensible object and a copy, “the effect or consequence of dream, reverie, imagination” (NB 843; also S 292). But for Spinoza, an idea is not a passive perception but “a concept of the mind that the mind forms” (E II D3, D3exp), an activity that constitutes a content. 
Even though Berkeley adopts a similar way of thinking about acts of perceiving and their objects (NB286), he fears that Spinoza’s doctrine of extended substance is easily misinterpreted by Hobbes, Henry More, Joseph Raphson, and Locke as endorsing the view that God is material (NB 290, 298, 825). Of course, such a misunderstanding is exactly what Spinoza had hoped to avoid by criticizing the Cartesian identification of extension with spatiality and matter. But Berkeley seems to accept the fact that the term extension has been so compromised that the only way to reintroduce it into a philosophic context is to identify it as a primary quality. 
For Spinoza at least, corporeal substance is not an infinitely large body or a combination of an infinite number of bodies, but rather extension or quantity “as it is understood in itself” (i.e., simply as the object or content of thought).  It is definitely not matter or space. Berkeley’s reason for thinking of Spinoza as an atheist is thus unfounded. Accordingly, his claim “tis nevertheless of great use to religion to take extension out of our idea of God and put a power in its place” (NB 298) is more properly directed at Descartes, Malebranche, Locke, and others who think of substance as a material or bodily. By substituting power for extension, Berkeley hopes to block the attempt to use extension as a principle of individuation distinct from God. The fact that Spinoza makes this same point in noting how “God’s power is his essence itself” (E I P34) indicates all the more Berkeley’s agreement with Spinoza.
Ultimately, Berkeley proposes that the best way to think about God or substance is as an immaterial principle, because he sees materiality as a principle of individuation distinct from God. Spinoza allows God or substance to be extended because he does not think of extension as matter or body but rather the objectivity of identification. Because neither Berkeley nor Spinoza allows for the possibility that anything apart from what substance constitutes is conceivable, neither of them allows for the possibility that anything apart from substance can exist.
26I have indicated (1) how understanding Spinoza’s doctrine of things benefits from a familiarity with Berkeley’s doctrine of ideas, (2) how Berkeley shares Spinoza’s view that substance is the activity, will, or power by which things become objects for minds, and (3) how both Spinoza and Berkeley can think that the substance of God is the same as the substance of nature without implying that God is identical to things in nature. This juxtaposition of Spinoza and Berkeley shifts attention away from assuming that ideas and minds have discrete identities to examining the process by which objects are differentiated as ideas in the simultaneous creation of minds. Such a shift indicates how Berkeley’s treatment of substance is much more like that of Spinoza, Leibniz, and Jonathan Edwards than that of Descartes, Malebranche, or Locke.
27I do not claim that Spinoza influenced Berkeley in the formulation of his fundamental philosophic views. I am only pointing out how Spinoza provides Berkeley with clues on what it means to speak about God and finite minds as pure act and spirits. It is possible that as Berkeley wrote part II of the Principles of Human Knowledge, he came to see how much of his doctrine of mind sounded like that of Spinoza. Faced with this realization, he could well have decided to heed his own advice not to offend the churchmen, and subsequently “lost” his manuscript.
See also DHP 213, TVV 6, and S 354. Abbreviations: Berkeley’s Principles of Human Knowledge [PHK section]; and Dialogues between Hylas and Philonous [DHP page] are from The Works of George Berkeley [W], ed. A. A. Luce and T. E. Jessop (9 vols.; London: Thomas Nelson, 1948-1957), vol. 2; Alciphron [Alc. dialogue and section], vol. 3; and Siris [S section], vol. 5. References to Berkeley’s Notebooks [NB entry], De Motu (Luce translation) [DM section], and Theory of Vision Vindicated and Explained [TVV section] are from Philosophical Works, ed. Michael. R. Ayers (Rutland, vt: Charles E. Tuttle, 1992).
See Sébastien Charles, “Berkeley and the Lumières: Misconception and Reconstruction,” in New Interpretations of Berkeley’s Thought, ed. Stephen H. Daniel (Amherst, ny: Humanity Books, 2008), 289.
See Charles S. Peirce, Collected Papers of Charles Sanders Peirce [CP], eds. Charles Hartshorne, Paul Weiss, and Arthur Burks (8 vols.; Cambridge: Harvard University Press, 1931-1958), V: 11; VI: 490.
See Henri Bergson, “Philosophical Intuition,” in The Creative Mind [CM], trans. Mabelle L. Andison (New York: Philosophical Library, 1946), 133; and Id., Creative Evolution, trans. Arthur Mitchell (New York: Random House, 1944), 383.
Spinoza’s Ethics [E, book, definition, explanation, proposition, and/or scholium], in The Collected Works of Spinoza [CW], vol. 1, ed. and trans. Edwin Curley (Princeton: Princeton University Press, 1985).
Cf. Bergson, Creative Evolution, 381; and Russell Ford, “Immanence and Method: Bergson’s Early Reading of Spinoza,” Southern Journal of Philosophy, 42 (2004), 181-183.
See Cornelis de Waal, “The Real Issue between Nominalism and Realism: Peirce and Berkeley Reconsidered,” Transactions of the Charles S. Peirce Society, 32 (1996), 425, 430-431.
Cf. Shannon Dea, “Firstness, Evolution and the Absolute in Peirce’s Spinoza,” Transactions of the Charles S. Peirce Society, 44 (2008), 622.
Charles S. Peirce, Pragmatism as a Principle and Method of Right Thinking, ed. Patricia A. Turrisi (Albany: State University of New York Press, 1997), 172; and Dea, “Firstness,” 603-610.
See Dea, “Firstness,” 622.
Cf. Stephen H. Daniel, “The Harmony of the Leibniz-Berkeley Juxtaposition,” in Leibniz and the English-Speaking World, ed. Stuart Brown and Pauline Phemister (Dordrecht: Springer, 2007), 166.
See René Descartes, Meditations on First Philosophy, Med. III, in The Philosophical Writings of Descartes [CSM], trans. John Cottingham, Robert Stoothoff, and Dugald Murdoch (2 vols.; New York: Cambridge University Press, 1984), 2: 30; and Principles of Philosophy, I: 51-52 (CSM, 1: 210).
See John Locke, An Essay concerning Human Understanding, ed. Peter H. Nidditch (New York: Oxford University Press, 1975), II.23.15-30.
Cf. Geneviève Brykman, “Berkeley, lecteur et critique de Spinoza,” Recherches sur le xviie siècle, 2 (1978), 180; Id., “Berkeley lecteur de Spinoza,” in Architectures de la raison: Mélanges offerts à Alexandre Matheron, éd. Pierre-François Moreau (Fontenay-aux-Roses, France: ens Éditions, 1996), 92; and Id., “Berkeley, Spinoza, and Radical Enlightenment,” in George Berkeley: Religion and Science in the Age of Enlightenment, ed. Silvia Parigi (Dordrecht: Springer, forthcoming).
Spinoza to Oldenburg, Nov.-Dec. 1675 (letter 73), in Benedictus de Spinoza, The Letters, trans. Samuel Shirley (Indianapolis: Hackett, 1995), 332. See also E I P18.
Berkeley, W 2: 84n; also see NB 652, 842. Cf. Stephen H. Daniel, “Edwards, Berkeley, and Ramist Logic,” Idealistic Studies, 31 (2001), 58-68.
See Stephen H. Daniel, “Berkeley’s Pantheistic Discourse,” International Journal for Philosophy of Religion, 49 (2001), 179-194.
Spinoza to Henry Oldenburg, Sept. 1661, Letter 2, CW 168.
Cf. Stephen H. Daniel, “Berkeley’s Stoic Notion of Spiritual Substance,” in New Interpretations, 216-224.
See also Berkeley to John Percival, 6 September 1710, in The Correspondence of George Berkeley and Sir John Percival, ed. Benjamin Rand (Cambridge: Cambridge up, 1914), 83-84.
See Spinoza to Simon de Vries, March (?) 1663, Letter 10, in CW 196. Cf. DHP 252.
Spinoza to Ehrenfried Walter von Tschirnhausen, 15 July 1676, Letter 83, in Complete Works by Benedictus de Spinoza [CWS], trans. Samuel Shirley, ed. Michael L. Morgan (Indianapolis: Hackett, 2002), 958.
See Spinoza to Oldenburg, September 1661, Letter 2, CW 165; and Spinoza to de Vries, March 1663, Letter 9, CW 195.
Cf. Samuel Shirley, Introduction, The Ethics…, ed. Seymour Feldman (Indianapolis: Hackett, 1991), 27: “An idea for Spinoza is an act of thought; it is almost a transitive verb having an object.”
Berkeley’s note about Spinoza’s letter to Oldenburg about extension (NB 844) is marked “P.”
See Spinoza to Lodewijk Meyer, April 1663, Letter 12, CW 202-203.