Written in May 2026 for Tufts PHIL152: History of Modern Philosophy
A major question among early modern philosophers was that of the stuff around us. What are thoughts, feelings, cells, or trees really made of? What causes them? Are there multiple different substances that compose these different aspects of reality, or does one substance underlie it all? French philosopher and mathematician René Descartes (as in the Cartesian plane) and Dutch philosopher Baruch Spinoza were both very much concerned with these metaphysical questions. Both thinkers are rationalists, believing in innate ideas in human beings and the potential to simply reason their way to certain truth about the world. First, I will explore each of their metaphysical systems, including their conceptions of both substance and God. Then, I will compare their respective applications of the definition of substance, explanation of causation between mind and body, and the relative plausibility of their metaphysical systems. I will conclude that while Descartes’ conclusions are more natural and intuitive, Spinoza is more coherent in his treatment of substance and causation, and his metaphysical picture is at least as conceptually plausible.
For Descartes, a substance is something that is wholly independent of any other thing, meaning it can be understood through itself (Principles, 51). If candidate substance A depends on some other thing, B, to be understood, then A is conceptually reducible to B and therefore not fundamental enough to qualify as a substance. A centaur, for example, can only be understood through the ideas of a horse and a man. Each substance has a principal property that constitutes its essence, without which it could not be understood. This principal property of a substance is its attribute. Thought and extension are the only two properties that qualify as attributes for Descartes. Thought defines the essence of minds, and extension defines the essence of bodies, or physically extended objects. All other properties, referred to as modes of a substance, are understood through these principal attributes. Modes encompass the qualities or states of a substance, meaning they only exist within substances (Principles, 53). The color blue, for example, only exists within the context of the thing that is being described as blue, not as an independent entity.
Beginning his search for truth in complete skepticism, Descartes’ first breakthrough is with respect to himself: even if God were to be an evil deceiver, he can be sure that he still exists as the one being deceived (Med II, 25). But what is he? He knows that he is doubting, thinking, and being deceived. Therefore, he exists as a thinking substance with the principal attribute of thought (Med II, 28). In continued introspection, Descartes finds the idea of an infinite and perfect God, asserting that such an idea could not possibly have been caused by his finite and imperfect mind. Here, Descartes employs what is now known as his Causal Adequacy Principle. He holds that the cause of an idea must actually contain what is found in the effect. If an idea of something exists, then the cause of that idea must truly be what that idea represents. For Descartes, to suppose that his idea of God doesn’t truly have an equivalently-existent cause is to suppose that an effect does not have a cause, but that is impossible. Therefore, God exists as the cause of everything, including Descartes’ idea of him (Med III, 50). Here, Descartes admits that God is the only substance to which the definition truly applies, and in doing so concedes that his definition of substance does not apply “univocally” (Principles, 51).
On the matter of extended objects, Descartes realizes that God chose to provide him with both sense perception and the ability for intelligent interpretation of these perceptions. As a consequence, God also caused Descartes’ propensity to perceive the world as extended and material. If extended objects didn’t exist, then God would be a deceiver, but we know this is not true. Therefore, bodies are the substance with the principal attribute of extension (Med VI, 79). As a result of his metaphysical positions regarding minds, bodies, and God, Descartes is classified as a substance dualist: minds and bodies are fundamentally different, and neither could ever be equated with God.
Spinoza is similarly rationalistic in his belief that the fundamental truth of the world around us is discernible through thought alone. In Part I of the Ethics, subtitled Proved in Geometrical Order, Spinoza begins with a set of Euclidean-style definitions and axioms and then demonstrates a series of propositions he believes necessarily follow. Spinoza takes the ideas of Descartes further than he was willing to take them. The result is monism, the surprising idea that reality is composed of a single substance, which Spinoza refers to as God. It is important to clarify that Spinoza rejects the traditional sense of God as an intelligent creator that chose to make the world maximally good. Spinoza’s God is impersonal and without will or purpose. He presents a God that simply is nature, which is to say everything.
By substance, Spinoza similarly means things that are conceived only through themselves, meaning truly independent of any other thing in all respects (Ethics, I D3). Substances have modes, which are properties of a substance that are, again, only within other things, either substances or other modes (Ethics, I D5). An attribute is what the intellect perceives as constituting the entire essence of a substance (Ethics, I D4). Like Descartes, Spinoza believes that thought and extension are attributes, but he goes further to say that there exist infinitely many more attributes, all presumably based on concepts we as humans cannot understand. The only two attributes humans are able to understand are thought and extension. God, for Spinoza, is officially defined as an “absolutely infinite being”, or a substance that contains all of the attributes, not merely an infinite number, each of which fully expresses his infinite essence (Ethics, I D6).
Furthermore, two substances cannot share an attribute. Any two distinct substances must differ in either attributes or modes. Differing by only modes reduces them to the same substance (Ethics, I P4). If they share an attribute, Spinoza insists that this attribute fully expresses the essence of each substance. In the same way the common attribute fully expresses the essence of the first substance, it will fully express the essence of the second. Therefore, the two substances have the specific essence that is expressed by this common attribute, meaning they are not distinct (Ethics, I P5). From this, Spinoza concludes that every substance must be infinite (Ethics, I P8). Something finite must be limited by something of the same nature: extended objects are finite because we can always imagine one greater, and the same goes for thoughts. However, he explains that “a body is not limited by a thought nor a thought by a body” (Ethics, I D2). Therefore, if a substance were to be finite, it would have to be limited by something of the same nature or essence, which is to say, the same attribute. Given that he has already barred the sharing of attributes, the infinite — and only — substance we are left with is God, containing all attributes. Each of God’s infinite attributes expresses his entire, infinite essence. This is Spinoza’s famous equivalence: Deus sive Natura, or a God that is nature.
To me, Spinoza’s treatment of substance is more compelling than Descartes’ because he holds true to the definition and accepts the result. Descartes is inconsistent in his use of the word substance and amends it as needed to align with his instincts. The two agree that substance must be completely independent of any other thing, but Descartes believes that minds, bodies, and God are all substances. In doing so, he admits that only God qualifies as truly independent, arguing that his definition simply applies differently to God than his creation. To Spinoza, this is a makeshift solution to whatever it is that motivates Descartes to believe that minds and bodies are both distinct and fundamental (Della Rocca, 42). Spinoza is uninterested in intuitive motivations based on our experiences and strictly holds to the agreed-upon definition. In the same way the sum of the lengths of any two sides of a triangle must always be greater than the length of the third, substance must always be truly independent.
In response, it is reasonable to question the idea that consistency is important in metaphysics at all. Who is to say that the world necessarily follows a strict set of definitions or rules and is always consistent? It is entirely possible that Descartes’ two-tiered picture of substance perfectly reflects the way things truly are. The problem I have raised here, however, is not one of metaphysical plausibility. Rather, it is that the use of the same word to mean different things without justification — especially a word like substance that bears so much weight — weakens the entire argument. Spinoza’s insistence on definitional consistency seeks to prevent the shifting of ideas to preserve personally-motivated or natural conclusions. Descartes’ willingness to change his terms as he sees fit makes his conclusions seem much more arbitrary and his system harder to follow, both of which render his metaphysical picture far less convincing.
In addition to inconsistency around the definition of substance, Descartes leaves a serious metaphysical problem regarding causal interactions between substances. Both he and Spinoza agree that substances are completely and conceptually separate; that is, neither depends on the other for existence or, importantly, in concept. Nevertheless, Descartes allows causal relations between substances. Given that our minds are able to causally interact with our bodies, as in the thoughts that cause my fingers to type these words, it is clear that Descartes’ metaphysical picture requires causal relations between substances. But in what ways can two things that have been established as having absolutely nothing in common ever interact? This is referred to as the Mind-Body Problem, an issue that continues to torment philosophers.
Spinoza holds that there cannot be conceptual relations nor causal relations between substances. To understand why he is firm on this, it is important to consider the nature of causation itself. I find that Michael Della Rocca, a well-regarded scholar of Spinoza, offers a particularly helpful interpretation: a causal relationship, at its core, is an explanation (Della Rocca, 44). A given effect exists in substance A because of a cause found in another substance, B. It is then clear that at least one idea of substance A depends on, or is explained by, substance B. Given that both A and B were defined as independent substances, this leads to an evident contradiction. I side with Spinoza: causal relationships are founded on, or derived from, conceptual relationships. Therefore, the mind and body cannot be as fundamentally separate as Descartes insists they are.
A natural objection to Spinoza’s proposed solution to the Mind-Body Problem is that he doesn’t really solve the problem, he just redefines the question by arguing there is no causal relationship at all, just one entity existing. He appears to have resolved one issue, but he has created another, which is arguably harder to understand. What does it mean for a substance to have infinitely many attributes, and how can a substance possibly have all of these attributes? If every one of the infinitely many attributes of the one true substance, God, can fully express every aspect of his essence, then for every mode of extension, there must exist a corresponding mode of thought (II P7). Therefore, Spinoza is not proposing that mind and body do causally interact as two modes of God, as that would separate the two. Spinoza argues that there is just a single event under one substance, namely God, and consequently no need for an explanation of any interaction between two entities. He insists that the accounts of an event through thought, extension, or any other attribute must each be describing the event in its entirety, meaning there is somehow no difference in the information or reality they convey.
This is Spinoza’s parallelism, a somewhat puzzling result of the metaphysical claim that there is only a single, infinitely-described substance. The generalization of this principle to not only thought and extension but also attributes that we cannot even begin to comprehend is a large step and is difficult to understand at all. However, Descartes’ world of two fundamentally different substances that mysteriously manage to interact is just as hard to accept. Unlike Spinoza, he has no reasonable account of how they interact, just an observation that they evidently do. While Descartes creates a more intuitive metaphysical structure by separating minds and bodies, his insistence on a causal relationship between these two substances still does not hold true to the definition of substance. Spinoza, on the other hand, does not waver on the definitions and propositions laid out in Ethics. The fact that his conclusions entail the unintuitive idea of parallelism is, to me, a price worth paying for a logically coherent account of how minds and bodies can be so connected.
Regarding the plausibility of Spinoza’s monism, it should be noted that I do not find much more concrete sense in the idea of an absolutely infinite God with infinitely many attributes or the idea of parallelism than anyone else does. However, this is God we are talking about. Simply because it is difficult to conceive Spinoza’s God is not enough to reject his arguments. There is no reason that metaphysical phenomena must make immediate sense or feel natural to us. After all, our perception is likely limited. Spinoza would say we are limited to thought and extension, and therefore we are unable to conceive of the other infinitely many attributes. In any sense, we do not know what we do not know. Along these lines, I am going to argue that upon deeper consideration, Spinoza’s metaphysical picture is at the very least as plausible as that of Descartes and potentially more natural than we may think.
I will present an argument from analogy and claim that physics has already led us to make unintuitive conclusions about the fundamental nature of reality similar to those proposed by Spinoza. Albert Einstein, the famous physicist, was often asked during lectures at American universities whether or not he believed in God. He put it this way: “I believe in Spinoza's God, who reveals himself in the harmony of all being, not in a God who concerns himself with the fate and actions of men” (Schlipp, 103). As science has progressed, our understanding of the world around us has become more and more fundamental. That is, we have empirically discovered what seem to be patterns, laws, and underlying structures that lead us to make informed claims about how the things around us actually work. Imagine a time before we understood that atoms are, for the purposes of this argument, the building blocks of all matter. Back then, it would seem utterly ridiculous that the water I see rushing in a river and taste when I drink is made of exactly the same stuff as the rocks that form into mountains or the air that floats imperceptibly all around us. The idea that objects and the seemingly empty space around them are composed of the same material is deeply unintuitive. However, we understand today, through consistent definitions and sound arguments, that each of these substances (in the scientific sense of the word) is made of the same fundamental stuff: atoms. There is a trend in our empirical observation that is leading us toward a unified picture of nature. This direction seems to me compatible with Spinoza’s conception of God, one who underlies and explains rather than one who decides and acts.
I admit the obvious objection that physics is more or less restricted to the realm of extension. Descartes, and even Spinoza, may object to this comparison in a rationalist sense, arguing that an idea such as God could not possibly be discovered through empirical observation. Undoubtedly, there is nothing in modern physics that attempts to explain the world through a being with infinitely many attributes. Similarly, mapping a fundamental particle onto Spinoza’s God may incorrectly suggest that his modes are specific components or subdivisions of God, which does not align with Spinoza’s conception of one indivisible substance. Moreover, the current standard model of particle physics suggests that there are multiple fundamental particles, each representing what seem to be different parts of the physical world. Only String Theory, which remains mathematically speculative and experimentally unconfirmed, proposes a single fundamental concept. I want to make it clear that I draw this comparison as a broad and purely conceptual analogy. Even if physics, as it stands today, is unable to prove Spinoza’s full picture, its development weakens the idea that reality must be divided into parts, which is the motivation behind dualism. Unintuitive conclusions are not unfamiliar to us: physics has shown before that underlying reality can defy our perceptions and assumptions. The progressive unification of empirical observation and the ultimate unification of monism are to some extent in agreement.
In addition to the fact that I do not find Spinoza’s nontraditional and unintuitive God any less plausible, I will go even further to say that the reflection of human-like qualities in the Cartesian God makes Descartes’ conception less plausible. Just as humans are one particular instance of, presumably, an infinitely large number of ways that beings can exist, there are infinitely many forms that God could take in terms of nature, appearance, ability, personality, or will. Of all the possible Gods, it seems unlikely that a true God would be in any way similar to the highly specific and limited nature of humans. Descartes’ God chooses, acts, wills, and designs. These are the characteristics of a human with a human mind — why should an infinite being with a definitionally different nature share them? Additionally, there is the question of why God has a will at all, or why he would choose to design this world in particular. For Spinoza, if God is truly perfect, what would he be seeking with a purpose? It must be something outside of him that he desires, but that would imply that God lacks something he wants. Lacking something that is presumed to be desirable would imply imperfection. For me, projecting these human characteristics onto God risks mistaking what has been defined as a universal and necessary being for a limited and contingent one.
Minds and bodies do seem so fundamentally different; this is a fact no one can deny. Descartes’ ideas around substance are easier to conceive, and people around the world believe in his God, or at least its concept. His idea that minds and bodies must be different is an easy conclusion to agree with. However, just as we now understand that air, water, and earth are all made of the same fundamental material, it remains possible that we could discover that minds are nothing more than an incredibly complex emergent property of the same material. That is why a connected picture of the world that explains the relationship we know must exist between mind and body and doesn’t treat these important aspects of our reality as exceptions to the definition of substance is more convincing. From the intricacies of a single cell to the formation of planets and black holes, the world we behold is full of staggering complexity and power. For Descartes, a being with perfect intelligence chose to design it all in more or less the same way an omniscient and omnipotent human would have. For Spinoza, we are merely particular expressions of God’s absolutely infinite power, which is to say the limitless harmony of Nature. I find the picture of a God that simply and necessarily is the world in all of its complexity, not of a God who stands apart from the world and shapes it, more faithful to both the demands of reason and the depth of what we observe.