“I think therefore I am”, arguably the most famous words in the history of philosophy. They were uttered by Rene Descartes as he set out to provide an unshakable foundation for the certainty of our knowledge. He is considered to be the father of modern philosophy because his framework toppled that of the scholastics and has been the dominant conceptual framework in which contemporary philosophical problems as well as the scientific enterprise have been approached. He ushered in the era of mechanical philosophy which has been the reigning paradigm in science. A paradigm case of our inherited intellectual framework from him is the mind-body problem. This essay will discuss Descartes’ substance dualism and how it came from his process of methodical doubt. I will discuss crucial concepts in Descartes’ metaphysics primarily substance, mind and matter and show why they are integral to substance dualism. Then I will discuss some implications and problems that emerge from substance dualism and show why Descartes’ metaphysics cannot solve mind-body problems. I will briefly argue that materialism with its commitment to the Cartesian framework cannot refute substance dualism as well. Finally I give a rather brief introduction to possible ways a return to scholastic metaphysics, provides a more robust metaphysical framework upon which the mind-body problem can be addressed.

Descartes ghost in the machine

Substance dualism is an idea that owes its birth to Rene Descartes; it claims that human beings are composed of two distinct substances, mind and body, which are radically different in that mind is immaterial and body is physical. Descartes was led to substance dualism through his process of methodical doubt. He states in his magnum opus, “I realized that if I wanted to establish anything in the sciences that was stable and likely to last, I needed—just once in my life—to demolish everything completely and start again from the foundations” (Descartes 2015: 1) . His aim was to establish a basis for knowledge that had complete certainty. He relied on the power of rationality rather than the empirical method and hence his epistemological method was that of the rationalists. He sought to emulate the certainty, logical rigour and method that mathematics had by starting with simple propositions and deductively building from them until reaching a final conclusion with absolute certainty (Descartes 2015:9).

The mechanical philosophy of Descartes

Descartes sets about building his epistemology by doing away with all beliefs that can be doubted. He doubts the reliability of his senses because they have often deceived him, one looks at a stick in water and thinks its bent, but in reality it is straight. He doubts the existence of an external world because it is possible that an evil demon could be deceiving him into thinking there is a particular world when there is none. Furthermore he adds that when  dreaming we have sensations that are indistinguishable from when we are awake even though there are no real objects causing them – demonstrating that sensations do not need our bodies (Coetser, et al. 2015:16). However one thing he could not be deceived about is that he is having a sensation – albeit a mistaken one nonetheless it remains a sensation. Finally he cannot doubt his own existence, because in order to doubt he must exist. He arrives at his ultimate certain fact – he exists as a thinking substance.

There are two concepts Descartes employs in his philosophy which are crucial to his substance dualism – (1) substance and (2) clear and distinct idea. According to Descartes a substance is something that exists and has particular essential attributes. Each substance has a principal property or essence that makes it the thing that it is (Coetser, et al. 2015:20). All other attributes are modes of the principle attribute (Rocca, 2002). To understand the principle attribute is to have a clear and distinct idea of the substance.

Descartes states that the principle attribute of bodies (matter) is extension and all other properties possessed by matter are modes of extension, “I realize that the following short list gives everything that I perceive vividly and clearly in them: •size, or extension in length, breadth and depth;•shape, which is a function of the boundaries of this extension;•position, which is a relation between various items possessing shape;• motion, or change in position” (Descarte, 2015) . Descartes’ metaphysics of substance and matter and mind marks a radical departure from scholastic metaphysics and leads to his mechanistic view of nature which laid down the central tenets for modern philosophy (Rocca, 2002). It logically follows from such a conception of matter that mental states which consist of consciousness, intentionality and rationality have no physical properties such as: extension in space, motion, shape and able to be perceived. I do think the fundamental error lies in Descartes’ departure from Scholastic metaphysics in his conception of substance and matter, a point I will argue further along.

Descartes has a clear and distinct idea of matter which is not identical to his clear and distinct idea of thinking. Because having a clear and distinct idea of something is to know the principle attribute of that substance it therefore follows that mind and body are two substances which a human being is composed of – mind is immaterial and body is material. Now this leaves Descartes with the problem of how such radical different substances can interact causally with one another. In what sense can mind and body be unified if they are such different substances?

The failure of materialism to resolve the mind-body problem

Approaches to resolve this problem have generally taken the form of either (1) Rejecting the material and declaring all of reality is mental – as idealism does; (2) Rejecting the mental and stating all of reality is physical and can be explained in physical terms; (3) Accepting the dualism and trying to explain how they could interact (Coetser, et al 2015:4). However the dominant and popular approach in contemporary philosophy is that of materialism. Materialism or physicalism is the idea that all of reality is material and that events and phenomena must be material or dependant completely on the material (Stoljar 2016).

Contemporary philosophy of mind divides mental states into three basic categories: Consciousness or qualia; intentionality; rationality (Feser 2013:3). Qualia which has to do with what Descartes would call sense experiences; “what it is like” to have the experience. How hot and cold feel, how redness looks, taste, and smell would be examples. Materialism cannot explain qualia precisely because sense perceptions are conceived not to exist in the objects but in the subjective mind.  It follows that the brain itself, being physical and extended cannot itself possess qualia anymore than a rock can. A second argument against materialism is that we could have a complete description of material facts about brains and neurons – and yet know nothing about what it is like to actually “feel pain” or “see red”[1].

Intentionality is the directedness, aboutness that mental states have. For example I can have thoughts about my lunch, Descartes, what happened yesterday. My mental states are about something else, some other state. If physical objects have no properties of intrinsic teleology, final causation as Aristotle conceived, or directedness – a claim Descartes rejected in his conception of matter then if follows logically that the brain itself being physical cannot in principle explain intentionality.

Rationality is the paradigm feature that distinguishes humans from all other animals. Our ability to grasp concepts, combine them and make judgements consistent with the laws of logic. Concepts are abstract and universal as opposed to concrete and particular. For example the concept of a triangle is abstract and universal in that it applies to all triangles that were ever drawn or will be drawn. If I draw a triangle it has a particular size and colour; if I picture a triangle in my imagination it has a particular size and colour. However my concept of a triangle does not have any size or colour precisely because it applies to all triangles. For Aristotle and Aquinas it is specifically the ability to grasp abstract universal concepts that separates the material from the immaterial (Feser 2013). Concepts and certain forms of thinking are determinate in a way that no physical thing is determinate. For example words are indeterminate in their meaning in that the same word such as “arms” can have different meanings – it can refer to the bodily organ or to guns. The word itself is indeterminate in its meaning – in that the word is completely compatible with different meanings. However the concept of an arm (bodily organ) is determinate – thinking of the concept of the arm as a bodily organ is incompatible with any other concept. It is impossible that I could be thinking of the concept of the arm as a bodily organ and at the same time it be possible that I am actually thinking of some other concept. The argument is that because rationality involves the ability to grasp abstract, universal and determinate concepts it cannot come from any faculty that is material.

The implications of Descartes mechanical philosophy necessarily lead to a dualism. Descartes rejects the classic Scholastic conception of a substance being a unified whole and accepts a materialistic and reductionist view of nature where the only properties matter has are those susceptible to mathematical descriptions. Only quantitative properties are said to inhere in matter. Qualitative ones, qualia such as redness, hot, warm, cold are generated in the mind and not in matter itself. Materialism leads to an absurdity by denying that the mind is immaterial but still accepting Descartes mechanical philosophy that matter is devoid of qualitative properties and final causality.

Further problems with Descartes’ metaphysics of nature

Descartes rejected the scholastic conception of substance and with it its ideas about matter and form. Substance dualism has no explanation for what principle unifies the mind and body. If that principle is either material or immaterial then it would need explanation itself as to how it could unify. Therefore there can be in principle no explanation for the unity of body and mind on substance dualism. Hence Descartes’ must appeal to God to provide the unity[2]. Rather than being an intrinsically unified whole a human being becomes a complex machine with a ghost inside that somehow moves the mechanical body parts.

The second problem is that for Descartes all bodies (matter) are the same substance, they have the same essential attribute of extension. However what accounts for the different kinds of bodies that exist – water, giraffes, etc? What principle confers on bodies their other properties that make them unique if their essential attribute is all the same? Take water, suppose some additional principle W makes water the kind of thing it is with properties X, clearly this principle W cannot be a mode of extension otherwise giraffes as well would have it in virtue of possessing the same essential attribute of extension. However, if principle W is not a mode of extension where does it come from? How does it ensure that properties X only occur in water and not in giraffes? Descartes would have to then say water has two essential attributes – that of being a body and that which makes it water[3]. This means that water is also a type of substance dualism. The general problem rises again what unites the two substances in water to form a unified object?

A third problem is that Descartes conception of matter is arbitrary. Descartes chose extension, length and breadth (quantifiable properties) as essential to matter and attributes like colour, taste (qualitative properties) as not being essential because they could not be expressed in mathematical language and that therefore they reside in the mind. This is mistaken because we can only know that some object is of a certain size by perceiving it as much as we come to know its colour by perception.

The metaphysics of Aristotle briefly

The scholastic conception of a substance which Descartes rejected summarised states:

“…a material substance consists of a substantial form added to prime matter which is an uncharacterized substratum of properties. The substantial form of a substance is that in the substance that accounts for the kinds of actions it could perform and the kinds of changes it could undergo. On this view, the substantial form of a substance is regarded as constituting its essence and thus, since different corporeal substances have different substantial forms, they have different essences as well.” (Rocca 2002).

Aristotle held that each distinct and individual thing, a substance was composed of form and matter. So for example a table is composed of wood and this would be the material parts which are then arranged into a specific structural configuration which would be the form. The wood is necessary for the table to exist however it alone is not sufficient because one could have a plastic or metallic table. The form is what distinguishes the table from a chair and give it is unique properties. However a table is artificial in that its form is artificial and extrinsic and derived from human agents. Natural substances have an intrinsic form which actualizes matter and gives the substance its essence, identity and unique capacities. The form is not simply the physical static shape but rather is an intrinsic principle from which the identity of a thing is derived It is “the intrinsic incomplete constituent principle in a substance which actualizes the potencies of matter and together with matter composes a definite material substance or natural body” (Oderberg). It is dynamic in that it determines how a specific substance changes over time such as how a human being develops from a foetus to a mature adult- it is the internal principle of organization of all the material parts through space and time. It is matter and form together which make up a whole and unified substance. Unlike a table the natural form is not derived from some external agent but is intrinsic to the natural substance. Therefore for the scholastics and Aristotle each natural substance such as water, humans, giraffes, protons are not reducible to their material parts- you cannot explain what a thing is on the basis of its material parts alone. The principle of form explains the uniqueness of each natural substance. Because of its form each natural substance has certain causal capacities and ends it is directed towards (efficient and final causality). An orange tree because of its form which organizes its material parts in a specific way has the capacity or power to bear orange fruits.

The Aristotelian metaphysical worldview is highly anti-reductionist. It conceives of the actual world as rich and enchanted with many basic properties that re simply not reducible to material parts. On an Aristotelian view it doesn’t make sense to try and find the form of a substance by dissecting and studying its material parts otherwise you would be thinking of form as some other kind of part, like mystical glue holding together the material parts – but it would still be a part nonetheless. Rather the form is the principle of unity that organizes the material parts into a coherent whole. Just like it would be nonsensical to try and find the form of the wooden table by analysing the wood’s chemical composition – rather the form is what makes a table the kind of thing by how the wood is arranged into that specific structure. On the Scholastics view a human being is a unified substance whose form consists of a rational capacity thereby making humans unique in possessing an immaterial form. Animals which lack the ability to grasp abstract and universal concepts, although conscious, would be material according to Aristotle. There was no mind-body problem prior to Descartes precisely because there was no mechanical reductionism – recall for Descartes all there is to matter is size and extension. However for the ancients and medieval philosophers matter consisted of more capacities and properties then what could be quantitatively analysed.

The scholastics framework provides a more consistent framework about substances in which difficult philosophical problems can be resolved. The significant point is that each kind of unique thing is a substance – a human being is a single substance and is not composed of two substances meaning there is no interaction problem. The substantial form of the human confers its capacity to reason, sense and digest food for example[4]. Descartes conflates an essential capacity humans possess with being human.  A human being does have an immaterial nature in that his rational faculties are directed towards concepts which are universal and abstract, rather than the concrete and particular (Feser 2013)[5].


In conclusion, Descartes method of doubt and desire to emulate the certainty of mathematics leads him firstly to reject the scholastics framework and adopts a faulty conception of substance, matter and mind. Any mechanistic philosophy will fail to reconcile the mental and the physical because in principle it defines the physical to exclude the mental. Physicalism will fail in principle to explain mental phenomena precisely because it broadly assumes Descartes’ mechanistic and reductionist metaphysics. It raises a general metaphysical problem in that objects on Descartes’ framework must at least have two substances. Descartes’ idea of bodies is also arbitrary in that quantifiable properties are essential and intrinsic while qualitative ones are not – this is influenced by his desire for mathematical certainty in philosophy. Whereas the scholastics’ framework of substance is radically different and more consistent in explaining what makes an object the thing it is. Only on a coherent metaphysical framework can philosophical progress be made and Descartes’ mechanistic modern philosophy provides no such foundation and leads to unavoidable problems of which the mind-body is a paradigm case in point.


[1] For further reading see Frank Jackson’s The knowledge argument where he uses a thought experiment about Mary a scientist who lives in a black and white room with complete knowledge about the physical facts of neurophysiology and vision. When Mary steps outside and sees a red apple she would have acquired a new fact – that of knowing what “red is like”. Therefore physicalism cannot be true because Mary had all the physical facts and still lacked that “redness” fact (Jackson 1982).

[2] Appealing to God also does not solve the problem because you end up with an artificial and imposed unity, rather than an essential and intrinsic unity. This led to Occasionalism which is the idea that God is the efficient cause of all motions and differentiation of matter because bodies themselves are inert. As the entry in Encyclopaedia of Philosophy states, “The true cause of both sensations and voluntary movements was God, who instituted laws for the union of mind and body and acted accordingly in particular instances.” (Doney 2006).

[3] The argument cannot be refuted by appealing to laws of physics. Appealing to the principle of chemical bonding between hydrogen and oxygen is irrelevant precisely because it is the fact that water has the property of being able to be produced by chemical bonding of hydrogen and oxygen. If all bodies have the same essence why does water have this unique ability of being produced by chemical bonding of Hydrogen and Oxygen while giraffes and stars which are essentially bodies as well do not? The argument could be illustrated using electrons or quarks which are considered elementary particles and therefore not composed of any parts- therefore one cannot appeal to the behaviour of the parts of electrons to explain its unique properties. See also (Oderberg 2011:90).

[4] See how David Oderberg develops the argument for hylemorphic dualism at length (Oderberg 2005)

[5] For the Scholastics our perceptions, sense experiences are material in that they are directed towards concrete particulars.  To be material is to be some concrete particular thing with a form directed towards some end.


Coetser, Y., Mkhwanazi, E. & Scott, C. D., 2015. Modern Philosophy Study Guide for PLS3702. Pretoria: University of South Africa.

Descarte, R., 2015. Meditations on First Philosophy, s.l.: s.n.

Descartes, R., 2015. Discourse on the Method, s.l.: s.n.

Doney, W., 2006. Cartesianism. In: D. M. Borchert, ed. Encyclopedia of Philosophy. 2nd ed. s.l.:Thomas Gale, pp. 53-59.

Feser, E., 2013. Kripke, Ross, and the Immaterial Aspects of Thought. American Catholic Philosophical Quarterly, 87(1), pp. 1-32.

Jackson, F., 1982. Epiphenomenal Qualia. The Philosophical Quarterly, 32(50), pp. 127-136.

Oderberg, D., 2005. HYLEMORPHIC DUALISM. Social Philosophy and Policy, 22(2), pp. 70-99.

Oderberg, D., 2011. Essence and Properties. Erkenntnis, 75(1), pp. 87-113.

Rocca, M. D., 2002. A Companion to Early Modern Philosophy, s.l.: Blackwell Publishing.

Stoljar, D., 2016. Physicalism, s.l.: Standford Encycopedia of Philosophy.