The notion of essence occupies a central place in Spinoza’s philosophy. It features prominently not only, as one may expect, in his metaphysics, but also in virtually all other parts of his system, including his philosophy of mind, his theory of affects, and even his political philosophy.1 In the Ethics alone, the term “essentia” appears 230 times.2 This is no coincidence. Essences play a crucial explanatory role at many critical junctures of Spinoza’s philosophy.3 Examples abound, so let me mention just four: (i) definitions are said “to explain the inmost essence of the thing” (TIE 95), (ii) God’s power is identified with God’s essence (in E1p34), (iii) the striving (conatus) of a thing is identified with the “actual essence” of the thing (in E3p7), and (iv) desire is defined as the essence of human beings, “insofar as it is conceived to be determined [. . .] to do something” (E3defaff1).

Even though essences are obviously quite important for Spinoza, it turns out to be surprisingly difficult to determine what exactly he takes them to be. What are essences for Spinoza? Are they perhaps concepts or ideas? Or are they instead something in things? And what precisely is their function? There are only a few passages where Spinoza treats essences as a topic in their own right, and these passages are rather condensed in content and style. He does not seem to consider it necessary to provide a detailed elaboration of the notion of essence. This can leave his readers rather confused. The aim of this paper is to counteract such confusion and to reconstruct Spinoza’s account of essence, as it appears in the Ethics. I believe that at least some of the confusion surrounding his account can be overcome and that Spinoza in fact has a conception of essence which is both geared to his needs and interestingly different from other accounts of essence.

The central claim of this paper is that, for Spinoza, the essences of non-divine, singular things—that is, the essences of finite modes—do not suffice to render these things fully conceivable. On this reading, we do not fully understand ‘what a thing is’ just by knowing its essence. This may come as a surprise. Many philosophers believe that it is precisely the job of the essence of X to tell us what X is. Spinoza is not one of them. There are, as we will see, systematic reasons for why Spinoza cannot adopt such a view of essences. One crucial aspect of my interpretation is that we have to take great care to distinguish Spinozistic essences on the one hand from Spinozistic concepts on the other, a distinction which is often overlooked. To be sure, essences and concepts are closely related for Spinoza, but simply identifying them is a mistake which conceals one of the most central aspects of Spinoza’s account of essence. (The tradition of identifying Spinozistic essences with Spinozistic concepts goes back at least to Harry Wolfson [see Wolfson 1934: 350]. Many contemporary Spinoza scholars still operate within this framework.4 This Wolfsonian tradition basically treats Spinozistic essences as if they were Leibnizian complete concepts—an assumption which I will argue is mistaken.)

The unusual metaphysical framework Spinoza operates in poses special challenges for reconstructing his account of essence. As a foretaste of what is to come, let me mention just one example. Spinoza sees causal and conceptual relations as much more closely related than most contemporary philosophers would feel comfortable with. On his view, if A causes B, then B is conceived through A. This seems to force us to include a whole lot in the essences of things, perhaps even their entire causal history. This threat of ‘overloading’ (which we will encounter in different guises) is going to be a theme throughout the paper. The problem with overloading essences is not so much its initial implausibility (it is Spinoza, after all, whom we are talking about), but rather that essences cannot do the work Spinoza intends them to do when they are overloaded. One might think, therefore, that Spinoza is in trouble. I am going to argue, though, that he successfully avoids the problem of overloading of essences and hence avoids being in trouble.

The plan for the paper is as follows: Section 2 introduces three different models of thinking about essences, which will help us structure the discussion. Section 3 contrasts individual with universal essences and clarifies why the focus of this paper is on the former. Section 4 explains why essences are not analyzed in modal terms by Spinoza. Section 5 introduces two different versions of the ‘overloading’ problem. Section 6 argues that Spinoza is aware of these problems and successfully avoids overloading essences. Section 7 concludes.

Starting with Plato and Aristotle, essences have played a central role throughout the history of Western philosophy. While many other concepts and tools of ancient and medieval inheritance were jettisoned during the early modern period, the notion of essence survived the upheaval of this time without many bruises (unlike, for example, the notions of formal cause or prime matter). In fact, essences still, or perhaps again, play an important role in contemporary metaphysics. Different philosophers, however, have taken different views on how essences should be understood. It is useful, therefore, to briefly survey some of the prevalent accounts of essence before we turn to elucidating Spinoza’s conception of essence.

The essential features of a thing are typically thought to be the very core features of that thing—features which somehow belong more intimately to the thing than its other, non-essential features. In addition to that, the essential features of a thing are often described as the features in virtue of which the thing is what it is (as we shall see later, however, Spinoza rejects this characterization of the essential). There are different ways to unpack the notion of essence. In an influential paper, Kit Fine has suggested to distinguish between two different accounts of essence. The first model conceives of essences in modal terms (call this the modal account of essence). On the modal account “an object [has] a property essentially just in case it is necessary that the object has the property” (1994: 3). Features are essential to a thing, then, just in case the thing cannot lack those features. On Fine’s diagnosis, the modal account was the dominant view throughout the second half of the twentieth century. The second model of understanding essences—the one which is favored by Fine himself—analyzes essences not in modal but in definitional terms (call this the definitional account of essence). As Fine himself puts it, “just as we may define a word, or say what it means, so we may define an object, or say what it is” (1994: 2). Features are essential to a thing, then, just in case they are mentioned in the (real) definition of the thing.5

Fine objects to the modal account that it miscategorizes some of the non-essential features of things as essential ones. He famously uses the example of singleton Socrates to show this. His argument runs roughly as follows:6 it surely is necessary that (if Socrates exists) Socrates belongs to singleton Socrates (for necessarily, the singleton exists if Socrates exists). On the modal account, however, it follows from this that Socrates is essentially such that he belongs to singleton Socrates. But this, Fine maintains, is simply false. Thus, the modal account fails to carve out the essential features of things on his view. (Whether or not the singleton Socrates case and other putative counterexamples to the modal account are ultimately successful is not our present concern. For our purposes, it is enough to have a basic grasp of how the singleton Socrates counterexample is supposed to work.)

In addition to the two accounts just presented there is at least one more that deserves our attention, which may be called the explanatory account of essence. On this view, the essential features of a thing are the explanatorily most fundamental ones, which cannot be explained by any other features the thing has. They are, in other words, “the object’s deepest explanatory properties” (Robertson & Atkins 2020: sect. 2). Accidental features, in contrast, are those which are less explanatorily fundamental. Note that the notion of explanation relevant here is that of (mind-independent) metaphysical explanation, which is a relation that holds between different states of affairs or facts, regardless of whether anyone uses this relation to actually provide an explanation.7

To sum up, there at least three different ways to unpack the notion of essence: the modal way, the definitional way, and the explanatory way. As we will see in the following sections, classifying Spinoza is no straightforward matter and the three models just introduced should not be seen as too rigid a framework. In fact, we will see that Spinoza’s account of essence is rather different from basically any other account suggested throughout the history of philosophy. With this caveat in mind, let us proceed to investigating Spinoza’s account of essence.

When Spinoza talks about essences, are the essences he is concerned with individual or universal? That Spinoza commits himself to individual essences is relatively uncontroversial. Individual essences belong uniquely to one individual thing and cannot be shared. Whether there are, in addition, also universal essences according to Spinoza—that is, essences which can be shared—is a much more controversial issue. Many commentators argue that Spinoza’s repudiation of universals simply amounts to a rejection of universal essences. On this view, Spinoza only allows for individual essences.8 Others suggest that Spinoza’s regular appeal to species essences like ‘horse’ or ‘human being’ should be taken at face value. On their view, Spinoza commits himself to there being universal essences (even though he might nonetheless deny the existence of Platonic universals).9 Fortunately, it is not necessary to resolve this scholarly dispute for the purposes of this paper.10 It will suffice to explain, first, why there is good reason to think that Spinoza assumes that there are individual essences, and second, why there is good reason to think that he has such individual essences in mind whenever he speaks of the essences of singular things.

The most important piece of evidence for both of these conclusions comes from the definition of essence at the beginning of book II of the Ethics, where Spinoza states:

This definition is puzzling in more than one respect. For now, let us focus on the concipi-formulations in the second clause. Crucially, Spinoza does not say, as one may expect, that everything that is needed to conceive of a thing belongs to the essence of this thing. Instead he says, and this is quite baffling, that a feature is essential just in case it is needed to conceive of the thing and cannot “be conceived without the thing.”12 Thus, for Spinoza the relation between a thing and its essence is symmetrical. As we will see later, the purpose of the second conjunct is to avoid the ‘overloading’ of essences and to exclude external factors from the essences of things.

At this point, what matters is that the symmetry of essence and thing is often thought to rule out universal essences and to only allow for individual essences.13 This is a very natural thought: universal essences, it would seem, can quite easily be conceived without the particular things corresponding to them. The universal essence of horse, for example, is not dependent on this or that particular horse. Apparently, then, the (universal) essence of horse can be “conceived without the thing,” contrary to what Spinoza says about essences in E2def2. This is why most commentators think that E2def2 only allows for individual essences, for which it is much easier to see that they can meet the symmetry requirement articulated in this definition.

As Karolina Hübner has recently suggested, however, this is not the only reading available. She rightly points out that Spinoza only talks about things (res) in E2def2, not about particular things. It could be that Spinoza has in mind things that “figure also at higher levels of generality” (2016: 64), not just particular or singular things. Therefore, Hübner continues, “E2def2 simply leaves undetermined the level of generality proper to the ‘essences’ and ‘things’ it invokes” (2016: 64–65). Even on this reading of E2def2, however, Spinoza is committed to the existence of individual essences. For whatever the exact scope of ‘res’ in the definition is, it surely includes singular things (see Hübner 2016: 65). Thus, Spinoza posits individual essences on all available readings of E2def2.14 Moreover, his definition rules out that singular things have universal essences; their essences have to be individual. For if we plug in a singular thing (res singularis) for the res in E2def2, the symmetry requirement dictates that the essence of that singular thing cannot be conceived without the thing. This rules out universal essences: only the individual essence of a singular thing X is such that it cannot be conceived without X. Thus, whenever Spinoza talks about the essence of a singular thing, he must be talking about its individual essence, not about a universal essence (this will become relevant later on).15 To sum up, Spinoza not only thinks that there are individual essences; he also assumes that the essences of singular things cannot but be individual (if Hübner is right, though, there may be non-singular things with universal essences for Spinoza).

Before I go on, let me briefly touch on another issue which has received some attention in the recent literature: the distinction between actual and formal essences (essentia actualis and essentia formalis). That Spinoza draws such a distinction is uncontroversial, but there is no consensus on what exactly this distinction amounts to. Some commentators argue that there is a realm of unactualized formal essences and that the formal essences exist independently of the actual ones.16 Actual essences are then often thought to be the actualizations or instantiations of formal essences.17 Mogens Lærke, however, has recently criticized such readings as too Platonizing. He instead suggests that the actual and the formal essence are different aspects of one and the same entity.18 I will not attempt to resolve this debate about actual and formal essences. Everything I have to say about essences in the remainder of the paper is intended to apply to both types of Spinozistic essences, regardless of how exactly the distinction between actual and formal essences is spelled out.

Spinoza subscribes to necessitarianism, according to which all truths are necessary truths.19 He writes: “In nature there is nothing contingent, but all things have been determined from the necessity of the divine nature to exist and produce an effect in a certain way” (E1p29). We are under the impression that there is contingency in the world merely “because of a defect of our knowledge” (E1p33d). That is, we (mistakenly) believe that things could be different from how they actually are only because we do not grasp that, and how, God necessitates everything that is going on in the world. On Spinoza’s necessitarian picture, then, the way things actually are is the only way things could have been.

Given Spinoza’s commitment to necessitarianism, it is relatively easy to see that the modal account of essence is not at his disposal. For him, each thing has all of its features necessarily. On the modal account, this results in each thing having all of its features essentially—which is highly implausible. Here is how implausible this is: If it is a necessary truth that Angela Merkel is (as of July 2021) chancellor of Germany, then my neighbor’s dog is necessarily such that Angela Merkel is chancellor. On the modal account, this entails that the dog is essentially such that Angela Merkel is chancellor, which is an absurd result.

We are confronted here with a first version of the problem of overloading. Since, for Spinoza, all the features of a thing belong to the thing necessarily, they all get built into the essence of the thing, if we follow the lead of the modal account. This ‘overloads’ the essence; we are forced to include a great deal too much in it. Hence, the essential cannot be distinguished from the non-essential in a meaningful way. As a result, analyzing Spinozistic essences in modal terms is futile, simply because there is far too much necessity in Spinoza’s system. (As a matter of fact, this version of the problem of overloading bears some similarity to the problem which the contemporary modal account faces according to Fine. On Fine’s view, we cannot carve out the essential features by appealing to necessity, because things have ‘too many’ necessary features for that. In Spinoza, this problem is greatly exacerbated because all features are necessary features.)

That Spinoza’s commitment to necessitarianism is incompatible with an analysis of essence in modal terms has been noticed before. Jonathan Bennett, for instance, thought that “[Spinoza’s] uses of the concept of a thing’s essence, meaning those of its properties which it could not possibly lack, are flattened into either falsehood or vacuous truth if there are no contingent truths, because then every property of every thing is essential to it” (1984: 114). The trouble of course only arises because Bennett simply presupposes that Spinoza subscribes to the modal account of essence (call this the modal interpretation). Fortunately, a more promising strategy is not far to seek: we simply need to give up the modal interpretation of Spinozistic essences.20 As several commentators have pointed out, there are very good textual reasons for doing so. Spinoza unambiguously commits himself to the existence of so-called propria—features a thing has necessarily, but not essentially.21 His acknowledgment of propria at once refutes the modal interpretation. Someone who analyzes essences in purely modal terms cannot accept such necessary and yet non-essential features.

Here are three representative passages, where Spinoza draws a distinction between essential features and propria:

In these texts, Spinoza quite clearly makes a distinction between the essence of a thing on the one hand and that which necessarily follows from the essence on the other. Employing traditional Aristotelian terminology, he calls the latter propria (or proprietates). What exactly is a proprium? The Aristotelian stock example of a human proprium is risibility, that is, the ability to laugh. Even though humans do not have this ability essentially, it necessarily follows from their essence that they have it. Humans are, on the Aristotelian view, essentially rational, and it is in virtue of their rationality that they have the ability to laugh. More generally, the essential features of a thing and its propria stand in an asymmetric explanatory relation to each other, so that the former account for the latter.

Given Spinoza’s stable commitment to propria, it is clear that the modal interpretation of essence needs to be rejected not just because it leads to the problem of overloading, but also for textual reasons. Propria are necessary, but non-essential, features of things. Such features are ruled out by the modal account of essence, on which all the necessary features of a thing are ipso facto essential to it. Since Spinoza explicitly acknowledges propria, we can conclude that he would not accept a modal analysis of essence.

Aside from the fact that Spinoza rejects the modal account, can we say anything else about his conception of essence at this stage? We certainly can. As we have seen, Spinoza sees definitions and essences as closely related. He says that a perfect definition explains “the inmost essence of the thing” (TIE 95) and that inferring the proprietates from the definition of a thing amounts to inferring them from its essence (E1p16d). These remarks suggest that Spinoza adopts a version of the definitional account of essence. This shouldn’t come as a surprise, given that connecting essences with definitions in this way is quite standard at Spinoza’s time. At least in this respect, he clearly draws on the Aristotelian tradition.22

Another thing to notice is that Spinoza combines the definitional with the explanatory account of essence. For him, the essences of things explain why those things have certain propria. As we have seen, he says in E1p16d that “the intellect infers from the given definition of any thing a number of properties [proprietates] that really do follow necessarily [necessario sequuntur] from it (that is, from the very essence of the thing).”23 It is not immediately clear what Spinoza means when he says that the propria ‘necessarily follow from’ the essence. Initially, one might think that Spinoza simply has a logical entailment relation in mind. In that case, the fact that a thing X has the essential features F and G logically entails that X also has the proprium H—presumably because the concept of the proprium H is already somehow included in the concepts of F and G. Yet this reading faces at least two problems. First, it is hard to see how the distinction between essences and propria can be upheld on this reading: if the concept of the proprium H is already included in the concept of the essence, this seems to make H essential to the thing.24 Second, there are many entailment relations which are not Spinozistic ‘following-from’ relations (presumably, the fact that X is risible entails that X is rational—but that X is rational surely does not, in Spinoza’s sense, ‘follow from’ the fact that X is risible).25

Thus, the Spinozistic ‘following-from’ relation should not be understood as an entailment relation (or at least it must be something more than that). The way Spinoza uses ‘following from’ strongly suggests that he has primarily a metaphysical relation in mind. As Don Garrett has argued (convincingly, I think), when Spinoza says that y follows from x he intends “to locate x specifically as a necessitating cause and ground of y” (Garrett 1991: 194). Construed this way, the Spinozistic ‘following-from’ relation is not too different from the contemporary grounding relation.26 We can thus characterize the relation between essence and propria in the following way: essences are metaphysically and explanatorily prior to the propria they give rise to; furthermore, the essential features of a thing necessitate the thing to have certain propria. (All of this is not too different from the Aristotelian conception of how essences and propria are related. Suárez, for example, maintains that the propria ‘emanate’ from the substantial form.27 Such emanative language is still present in the Ethics as well.28)

To be sure, assuming that the essential features of a thing and its propria stand in an asymmetrical explanatory relation is by no means unusual. On the contrary, Spinoza seems to simply adopt the standard Aristotelian conception of how propria are related to essences. That Spinoza accepts this aspect of the Aristotelian conception of essence is articulated very succinctly by Thomas Ward:

We can add to Ward’s diagnosis that Spinoza also inherits the link between essences and definitions from the Aristotelian tradition. The only deviation from the orthodox view which we have detected so far is that Spinoza’s focus is (perhaps exclusively) on individual essences, not on universal essences.

So far, so good. Can we conclude that Spinoza by and large adopts an Aristotelian conception of essence, as several commentators have recently suggested? I think such a conclusion would be premature. To see why, we need to turn our attention to what I label the problem of overloading. In the last section, we have already encountered a first variant of this problem: on the modal interpretation, Spinoza is forced to integrate literally all the features a thing has into its essence, which results in this essence being ‘overloaded.’ This version of the overloading problem could be resolved easily because it was based on a mistaken interpretation of Spinoza’s account of essence. But the trouble does not stop here. I will now introduce two other versions of the problem of overloading, which will prove to be more worrisome.

The first problem has to do with Spinoza’s views on causation. At the outset of the Ethics, he declares: “The cognition (cognitio) of an effect depends on, and involves, the cognition of its cause” (E1ax4, translation modified).29 That the cognition of the effect ‘involves’ the cognition of the cause means (given Spinoza’s use of involvere in such contexts30) that cause and effect are conceptually related. Thus, if A causes B, then B is conceived through A.31 In order to have the concept of B one needs to have the concept of A. Given that effects are conceived through their causes, for Spinoza, it is quite understandable that some commentators have reached the conclusion that the causes of a thing must be included in its essence. Michael Della Rocca, for example, writes that “a thing’s causes are, for Spinoza, built in to its essence” (2008: 94).32 This interpretation is based upon a very natural line of thought: If the essence of a thing is supposed to tell us ‘what the thing is,’ and if a thing is conceived through its causes, then it seems unavoidable that the causes must feature in the essence. Otherwise, the essence of a thing would not enable one to conceive of that thing (and this seems to be the job of essences after all). This reading also seems to find some support in Spinoza’s texts. In Letter 60, for instance, he writes that “the idea or definition of a thing should express its efficient cause.”33

However attractive the picture just sketched may be, it also faces some severe problems. On the present suggestion, the essence of a thing X presumably contains information about X’s entire causal history. E1ax4 dictates that X’s immediate cause is conceived through its cause, which in turn is conceived through its cause, and so on. Thus, to fully conceive of X (i.e., to fully grasp what X is), we need to conceive of the entire causal chain leading up to X. As a result, we seem to be forced to include in X’s essence X’s entire causal history. Some event in the distant past, for example, which happens to feature in the causal history of my neighbor’s dog, thus seems to belong to the essence of my neighbor’s dog.

While counterintuitive, this in itself may not be a problem (Spinoza has many counterintuitive things to say after all). What is a problem, however, is that Spinoza explicitly opposes building the causes of things into their essences. In the following three passages, he relies on a clear-cut distinction between the essence of a thing on the one hand and its external causes on the other:

As is clear from these texts, there are several contexts where Spinoza carefully distinguishes between the essence of a thing and its external causes.35 In E3p4d, he says (and this sounds quite natural) that this amounts to a distinction between “the thing itself” and its external causes. All of this suggests that for Spinoza the essential features of a thing are intrinsic to the thing (a point I will return to soon). If this is so, however, then the (external) causes of a thing presumably would have to be excluded from its essence. We thus seem to have reached an impasse. On the one hand, things are conceived through their causes, which suggests that the causes need to be built into the essences of things. On the other hand, Spinoza explicitly excludes the causes of a thing from its essence. The problem of overloading thus reappears in a new guise. This time it is the close connection between causation and conception which causes the trouble.

To make things even worse, the problem of overloading appears in yet another guise. Here is how: For Spinoza, a mode is something “which is in another through which it is also conceived” (E1def5). Ultimately, he thinks, all singular things (i.e., all finite modes) are conceived through God. This leads him to say, among other things, that the “[idea] of each singular thing which actually exists, necessarily involves an eternal and infinite essence of God” (E2p45). Thus, Spinoza seems to commit himself to the view that God’s essence is included in the essence of each singular thing. As Martin Lin puts it, “for Spinoza, [singular] essences cannot exclude information about God” (2012: 438). This is a very natural conclusion indeed (note, however, that in E2p45 Spinoza only talks about the ideas of singular things, not about their essences).

Natural as it may be, it is not what Spinoza in fact says. He explicitly excludes information about God from the essences of singular things. In E2p37, he says that “[w]hat is common to all things (on this see L2, above) [. . .] does not constitute the essence of any singular thing.” The lemma Spinoza refers us to reads: “all bodies agree in that they involve the concept of one and the same attribute” (E2lem2). Hence, what Spinoza tells us in E2p37 is that the attribute under which a thing is conceived is not part of the essence of that thing. Since attributes are constitutive of God’s essence (see E1def4), it follows from this that the divine essence is excluded from the essences of singular things as well.36

At this point, one may wonder why Spinoza is so wary of overloading essences. Why is it so important for him that the essential features of a thing are internal to that thing?37 To answer this question, we must consider what role essences play in Spinoza’s psychology and in his moral philosophy (as they are presented in parts III–V of the Ethics). One key idea there is that human beings are active and powerful (as well as free and happy!) insofar as they act out of their own nature or essence; they are passive and impotent (as well as unfree and sad!), in contrast, insofar as they are acted upon by external forces—that is, insofar as they experience passions. In E4p23d, for example, Spinoza says: “Insofar as a man is determined to act from the fact that he has inadequate ideas, he is acted on (by IIIp1), that is, (by IIId1–2) he does something which cannot be perceived through his essence alone” (my emphasis). One important step in becoming freer and happier, Spinoza thinks, is to understand that our passions are caused not by our own nature, but by something external to us, which often is beyond our control. Whatever the details of Spinoza’s psychology and moral philosophy are, it is clearly very important for him to distinguish between those features which things (especially humans) have through themselves—in virtue of their own essences—and those features which are ultimately due to something which is external to them. This explains, I think, why it is crucial for Spinoza to avoid the overloading of essences. If he did not, he could not employ the notion of essence in the way he does in parts III–V of the Ethics.

Let me sum up. One can easily get the impression that Spinoza’s remarks on what belongs to the essences of singular things are hard to square with his remarks on what is needed in order to conceive of those things. On the one hand, he explicitly excludes quite a bit of information from the essences of singular things (namely, information about the causes of those things and information about the divine essence). On the other hand, he seems to be committed to making essences maximally inclusive. So, have we indeed reached an impasse? In the next section, I will argue that we have not. Once we see how Spinozistic essences differ from Spinozistic concepts, we will be able to resolve the apparent impasse.

Many philosophers, among them the Aristotelians, hold that the essence of a thing is supposed to tell us ‘what the thing is.’ On this view, if A is needed for conceiving of B, then A must be included in B’s essence. I will now argue that Spinoza does not share this understanding of essence. Although his account builds upon the Aristotelian view in some important respects, it also diverges from this tradition to a far greater extent than is usually acknowledged.

In order to make progress in understanding Spinoza’s account of essence, we need to gain a better grasp of how Spinozistic essences are related to Spinozistic concepts. It is often assumed that the former are analyzable in terms of the latter. Harry Wolfson, for example, writes: “By ‘essence’ [Spinoza] means the concept of a thing which may or may not have existence outside our mind” (1934: 350). To cite a more recent example, Samuel Newlands also sees essences and concepts as closely linked. He suggests that Spinozistic essences are similar to Leibnizian complete concepts.38 On my view, the relation between essences and concepts is not as straightforward as often suggested. I think that, for Spinoza, the essence of a thing is not simply the concept of that thing.

One reason why essences cannot be identical to concepts on Spinoza’s view is that he takes them to belong to two distinct metaphysical realms: essences are something in things whereas concepts are mental representations of those things.39 In E2def3—the definition of ‘idea’—Spinoza identifies concepts with ideas, and he uses the terms ‘concept’ and ‘idea’ interchangeably elsewhere too (e.g., E2p49d). This suggests that concepts are modes of the attribute of thought (and indeed this seems to be the only option in Spinoza’s overall framework).40 Essences, in contrast, are something in the things they are essences of. This becomes clear in the so-called Physical Digression in part II of the Ethics, where Spinoza identifies the nature of a singular body—that is, its essence—with a particular “pattern [ratio] of motion and rest” (E2p13slem1). A pattern of motion and rest is not a concept of course, but a feature of the body! This shows that essences and concepts cannot be identical for Spinoza.

In addition to being metaphysically distinct, Spinozistic essences differ from Spinozistic concepts in another important respect: the concepts of singular things encode more information than the essences of singular things. To see that, let us start with a remark from the end of part II of the Ethics. In E2p49d, Spinoza writes: “to say that A must involve the concept of B is the same as to say that A cannot be conceived without B.”41 Plugging in a particular finite mode for A, this yields that the concept of the mode involves the concepts of its causes as well as the concept of God (because mode A cannot be conceived without those things). Thus, concepts are maximally inclusive: the concept of mode A contains everything that is needed in order to conceive of mode A, which is a whole lot according to Spinoza. We can infer from this that Spinozistic concepts are rather similar to Leibnizian complete concepts.42 According to both philosophers, the concept of a thing allows one to deduce from it all the features a thing has. (To some, this may sound like an absurdly strong rationalist doctrine. But this is certainly Spinoza’s and Leibniz’s view.)

Now, one may expect Spinoza to say the same thing about essences. But he doesn’t. To the contrary, he explicitly denies that essences are maximally inclusive in the way concepts are. In a passage that is key for understanding his account of essence, and which is often overlooked, he elaborates on the definition of essence in E2def2 in the following way:

Spinoza here unambiguously states that not everything that is needed to conceive of a thing also belongs to the essence of the thing. From E2p49d we know that concepts are such that they make the things they are concepts of fully conceivable. Taken together, these claims suggest that essences encode less information than concepts. As a matter of fact, Spinoza makes clear that this is precisely the point behind the somewhat odd-sounding symmetrical phrases in E2def2. The second clause of the definition (specifying that the essence of a thing is not merely that without which the thing cannot be conceived, but also “what can neither be nor be conceived without the thing”) is meant to limit the extent of what is to be included in an essence. Unlike the concept of a (singular) thing, then, its essence is not such that knowing it enables one to fully conceive of the thing. It straightforwardly follows from this that at least in the case of singular things, their essences do not render those things fully conceivable.

This is a rather peculiar result. One might even worry that Spinoza is saying something outright incoherent.43 On the standard conception of essence, the essential features of a thing are those features in virtue of which the thing is what it is. It would seem, then, that knowing the essence of a thing automatically enables one to conceive of the thing, and that one cannot fail to know what the thing is if one knows its essence. So what is going on? Apparently, Spinoza outright rejects the standard conception of essence. For him, the essences of singular things do not make those things fully conceivable and they do not tell us what the things are. The essential features of a thing are thus not identical to those features in virtue of which the thing is what it is (although they are certainly among them). There is a sense, then, in which Spinoza gives up the traditional notion of essence altogether and replaces it with something rather different. His break with the Aristotelian tradition of thinking about essences is thus a lot more decisive than typically assumed. To be sure, Spinoza certainly holds that understanding the essences of singular things makes these things partially conceivable and contributes to our understanding of what these things are. Full conceivability and a full understanding of what things are, however, can only be achieved by looking beyond their essences.44

Returning to E2p10s2, Spinoza there makes clear that E2def2 is intentionally designed to avoid the problem of overloading. In fact, Spinoza deals with one version of this problem explicitly there: he emphasizes that “singular things can neither be nor be conceived without God, and nevertheless, God does not pertain to their essence” (E2p10s2). And just two paragraphs earlier, he criticizes philosophers who endorse a conception of essence which he thinks is overly inclusive:

This passage once more shows that Spinoza is very aware of the problem of overloading. He thinks that it is mistake to write too much into the essences of singular things (and the philosophers he attacks here may very well be the Aristotelians). His antidotes to overloading are the symmetrical clauses in E2def2.

That he wants to exclude not only God, but also the external causes of things from their essences becomes clear in E1p33s1, where Spinoza says that “a thing’s existence follows necessarily either from its essence or from a given efficient cause.”45 The causal case is exactly parallel to the case of God, and the strategy Spinoza outlines in E2p10s2 applies equally to both cases. A thing cannot be conceived without its causes, so its concept includes (the concepts of) those causes. The essence of the thing, however, contains no information about the causal history of the thing. Essences are not maximally inclusive in the way concepts are. Against this reading of E1p33s and similar passages, one might object that Spinoza could refer to universal essences there. Perhaps his intention is only to exclude the causal history of a thing from the universal essence of that thing, but not from its individual essence. In that case, individual essences could still be maximally inclusive, in the same way concepts are.46 I do not think, however, that this alternative reading of the passages in question (E1p33s, E3p4d, E4p20s, etc.) is very plausible. In all those passages, Spinoza is evidently concerned with singular things (he talks about things which feature in causal chains, which can be destroyed, and so on). And as we have seen in Section 3, the symmetry requirement that falls out of Spinoza’s definition of essence in E2def2 strongly suggests that the essences of singular things are always individual. Given that, it is unlikely that universal essences play any role in those passages.

It has become clear that Spinoza is not willing to relinquish the idea that the essential features of a thing belong to the thing itself and are internal to it.47 That this is so is crucial for how he employs the notion of essence in his psychology and in his moral philosophy. What features a (singular) thing has depends on two things—(i) what the thing is like in itself, and (ii) its environment—and Spinoza evidently wants to reserve the term ‘essence’ for the core internal features.48 Thus, a Spinozistic essence only captures a thing’s own contribution to its makeup; external factors are excluded from the essence.49 This in itself is of course rather natural and unremarkable. The Aristotelians, and many others, would be in full agreement. In the context of Spinoza’s system, however, where singular things conceptually depend both on God and on external causes, this leads to the peculiar-sounding result that the essences of things do not make these things fully conceivable. Hence the drifting apart of essences and concepts. (The only exception is the divine substance, in which case there is no such divergence between essence and concept. The reason is, of course, that there is nothing external to God through which he is conceived. God is conceived entirely through himself, so that his concept and his essence are on a par and encode the same amount of information.50)

With this in mind, let us briefly return to Spinoza’s definition of essence. The task of this definition is to carve out precisely those features which are necessary to conceive of the thing and which are internal to the thing. The symmetrical phrases in E2def2 accomplish just that. Spinoza does not include everything that is necessary to conceive of a thing in its essence, but only those features which also “[cannot] be conceived without the thing”—and these are precisely those features which are internal to the thing (God and the external causes of the thing, in contrast, can be conceived without the thing).

It may be helpful to contrast Spinoza’s account of essence with Locke’s, who characterizes (real) essences as follows:

For Locke, the essence of a thing is (i) supposed to carve out the features in virtue of which the thing is what it is (or, as Locke says, “whereby it is, what it is”), and (ii) to do so in terms of the thing’s internal structure or constitution. (This account is structurally similar to the Aristotelian theory. While for Aristotelians the inner structure of things is determined by their substantial forms, for Locke it is a particular arrangement of corpuscles.)

Spinoza’s qualm with Lockean (and Aristotelian) accounts of essence is that he thinks that (i) and (ii) cannot be pulled off together. That in virtue of which a thing is what it is (“whereby it is, what it is”) includes God and the causal history of the thing, on his view. He thinks that we simply cannot fully explain what a thing is just by unveiling its internal constitution. As a result, Spinoza has to reject either that essences make reference only to the internal structure of things or that essences render things fully conceivable. As we have seen, he chooses the latter option. Of course, singular things can be fully explained on Spinoza’s view (he adheres to the Principle of Sufficient Reason after all; see E1p11d2). This can only be achieved, however, by employing (maximally inclusive) concepts.

We have seen that the distinction between the inner structure of a thing on the one hand and external factors influencing the thing on the other hand plays an important role for Spinoza’s account of essence. One may wonder, however, whether Spinoza has the resources to draw such a distinction in the first place.51 Recall that in order to conceive of what a singular thing X is, one needs to conceive of X’s entire causal history. Given this, how are we going to separate X’s internal features from X’s external features? For Spinoza, X’s causes are constitutive of what X is. It would thus seem that there is not even a thing X one can think about independently of those causes. So how do we decide in a principled way which ones of X’s features count as internal and which ones count as external?

To answer this question, let me begin by drawing a distinction between conceiving of X on the one hand and conceiving of what X is on the other. On Spinoza’s view, it is possible to have some conception (albeit an incomplete one) of a singular thing X without, strictly speaking, knowing what X is. In such a case, we of course only partially conceive of X, not fully. This is something which happens all the time according to Spinoza, given that the bar for full conceivability is extremely high.52 (That there are cases in which we conceive of X without conceiving of what X is also makes sense independently of Spinoza’s framework. If I encounter an animal which is unknown to me, I can nonetheless form some conception of this animal without knowing what it is.)

To see how this may help with distinguishing between the internal and the external features of singular things, consider Spinoza’s account of metaphysical individuation. In the case of bodies—which is the only case he discusses—he famously argues that they are “distinguished from one another by reason of motion and rest, speed and slowness, and not by reason of substance” (E2p13slem1). Thus, it is a particular pattern of motion and rest which accounts for the individuality of a singular body.53 We are now in a position to see how Spinoza is able to distinguish between the internal and the external features of bodies. Importantly, one can individuate a body X without knowing X’s entire causal history and thus without knowing what X is. As a result, one can also determine X’s boundaries without (strictly speaking) knowing what X is. Distinguishing the internal from the external features now becomes a straightforward geometrical matter: whatever is literally inside X’s boundaries counts as an internal feature of X; whatever is outside of X’s boundaries counts as external. Given Spinoza’s doctrine of parallelism—the doctrine that the “order and connection of ideas is the same as the order and connection of things” (E2p7)—it is clear that there must be analogous distinctions between what is internal and what is external for all the other attributes as well.

Let us wrap up this section by situating Spinoza’s account of essence in his broader metaphysical framework. That Spinoza relinquishes the idea that the essence of a thing states ‘what the thing is’ certainly seems peculiar. But perhaps this is exactly how it should be in the Spinozistic universe. For Spinoza, singular things are not substances, they are finite modes. Modes are not conceived through themselves, but only through the substance they are modes of. Given this, it should not come as a surprise that the essences of singular things do not make these things fully conceivable. This is simply due to the fact that modes are only in part conceived through themselves. The essences of those modes pick out just this part.

Let me sum up. As we have seen, Spinoza rejects the modal account of essence and instead adopts a version of the definitional account which he combines with the explanatory account. On his view, the essential features of a thing are its definitional features and they are metaphysically and explanatorily prior to the propria of the thing. In this respect he is an associate of the Aristotelian tradition, even though his focus is on individual essences, not on universal ones. As I have argued, however, the extent to which Spinoza’s ultimate conception of essence differs from the Aristotelian one is much greater than usually assumed. Because he wants to avoid the problem of overloading (i.e., the problem of including too much in essences), he concludes that the essences of singular things do not render these things fully conceivable. This is due to the fact that neither God nor the causal history of a singular thing feature in its essence. Because both God and the causal history are needed to fully conceive of singular things, we do not fully grasp what a thing is just by grasping its essence on Spinoza’s view. There is a sense, then, in which Spinoza jettisons the traditional notion of essence and replaces it with something quite different. The essential features of a singular thing are not the features in virtue of which the thing is what it is (even though they are among those features and contribute to what the thing is). We have also seen that one must carefully distinguish between Spinozistic essences and Spinozistic concepts. For one thing, essences are something in things, so essences cannot be identical to concepts. Moreover, concepts—unlike essences—do render the things they are concepts of fully conceivable. They are thus much more inclusive than essences and contain information about God and about the causes of the things. Essences, in contrast, encode significantly less information, so that they do not become overloaded.