A paper on Descartes’ mind – body problem by Philosopher Simon Valkering.


The word aporia is derived from the Greek word aporos1, which means without a passage. A poros is, in turn, a means of passing a river by way of a path or a bridge that connects the two riverbanks and is used to carry goods or livestock across the river. Aporos is the absence of such a path or bridge between two riverbanks and puts one in the awkward position of being able to see the other side of the river (which one tries to reach), while standing there with livestock or goods, but not being able to reach it. This is a very concrete problem. The word aporia is the abstract noun that is derived from aporos1 and is used to denote the difficulty 2 that one experiences when there cannot be found a solution to or an explanation for a problem that is posed. In this case too, the direction one has to take in order to come up with a solution to the problem is known, but the link, the path that actually enables one to take that direction is absent.

One problem 3 that often leads contemporary philosophers to an aporetic state is the so called mind-body problem that is based on the thought that reality is fundamentally divided into two distinct substances of a different nature, which, despite them not having any properties in common, mutually interact with and affect each other and form a unity in the human being. Although these two distinct substances can be conceived of, the way in which the interaction between these two is established is, especially in the light of contemporary Physicalism, hard (if not impossible) to conceive of; the bridge-principle that establishes and explains their inter-relatedness is missing. We could say that the mind and the body are the riverbanks of one of the vast rivers that flow through contemporary philosophy and leaves some philosophers, without a bridge to cross, standing on the one side and staring to the other with their suitcases full of ideas.

René Descartes is, among contemporary philosophers, widely considered to be the one who introduced the substance dualism of mind and body in modern philosophy and thus as the father of the mind-body problem. Although the substance dualism that is introduced by Descartes indeed was quite a novelty 4and had a key role in his philosophy, there was – for as far as the philosophical concerns that are presented in his writings – no problem for Descartes with this distinction and the inter-relatedness of the two substances, because he had his bridge-principle. To him this distinction was instead a (part of the) solution to a problem with which he himself was confronted.5 If this was not a problem for Descartes, that means that it has become a problem later on and that something has changed in the conception of the dualism in relation to other philosophical concerns.

In this paper, I will discuss the appropriation of the Cartesian substance dualism in a branch of contemporary philosophy,6 argue that the contemporary appropriation of Descartes’ dualism deviates from Descartes’ own conception of this dualism and that the problem that arises from the contemporary conception is not (necessarily) a problem on account of Descartes’ writings, but rather on account of the appropriated version of the dualism, which is somewhat problematic. In order to do so, I will first argue that contemporary philosophy has adopted only a part of Descartes’ metaphysical scheme – the part that consists of the substance dualism – but not the whole and that, by doing so, it is confronted with a problem that is not (necessarily) a problem if Descartes’ whole scheme is taken into account. Secondly I will argue that the conception of the dualism in contemporary philosophy is too strong, because a consistent critique on Descartes’ philosophy in line with the principles of contemporary philosophy weakens the clear-cut distinction between the two substances and leaves contemporary philosophers with conceptual unclarities regarding the nature of the substances (and thus their distinction).

The Metaphysical Scheme of Descartes.

Descartes is often referred to as the philosopher who left us with a radical bifurcation of reality, by dividing the metaphysical realm into two distinct substances, notably res cogitans and res exstensa. 7 An important source for this distinction is Descartes’ Meditations and especially the second meditation in which Descartes proves the existence of the res cogitans, on grounds of the method of radical doubt, and claims that it is distinguishable from the material substance of the body (bodies in general) of which he can only judge that it has extension. 8 Subsequently, in the sixth meditation, Descartes proves the existence of material substances as existing apart from the thinking substance and finalizes the distinction between these two kinds of substances which, nevertheless, are unified and interact with each other in the human body.9 From this account it can indeed be concluded that Descartes divides reality into two distinct substances and this is the metaphysical scheme that is often ascribed to Descartes and adopted by contemporary philosophers, leading to the question of how the interaction between these two substances is possible, if indeed they are radically different and distinct. The question of interaction is, in any case, a legitimate question and was also addressed by Descartes. 10. But his answers to this question are no longer considered to be satisfying or legitimate answers, because they appeal to a God-principle in order to solve the problem. And so, the question of interaction (the mind-body problem) remains to be the focus of philosophical debate.11

The persistency of this question, however, is largely based on the tendency to take the substance dualism in Descartes’ writings and the metaphysical bifurcation that comes with it as fundamental point of departure. Because of this point of departure and the subsequent claim that the interaction between the two domains of reality cannot be explained, ‘Cartesian dualism’ is often rejected in contemporary philosophy and replaced for a Physicalist approach.12 Jaegwon Kim, for instance, explains the mind-body dualism of Descartes as comprising “two metaphysically independent spheres existing side by side” and he then argues that this position is unattainable, because the radical distinction and yet mutual interaction between these domains violates the causal closedness of each domain respectively.13 This is, according to Kim, especially a problem with respect to the aims of scientific research today that seeks to secure the causal closure of the physical domain in order to keep the possibility of a physical theory of everything alive.14 And so Descartes’ writings are dismissed on grounds of presenting a reality that exists of two radically distinct domains that cannot interact with each other without violating the causal closedness of each domain. The argument of causal closure principle regarding each domain and, in particular, the causal closure of the physical domain is one of the strongest claims (if not the only one) against Descartes’ dualism15, but its legitimacy and force can (and should) be questioned.

The claim that Descartes initially divides reality into two separate domains is indeed correct, but the idea that this division is of a radical and irreducible nature is undue and the dismissal of Descartes’ position on grounds thereof too rash, because this conception of Descartes’ dualism stems from an incomplete appropriation of his metaphysical scheme. Besides introducing the distinction between res cogitans and res extensa, Descartes pays considerable attention in the Meditations to prove the existence of God, 16 through whom he establishes the inner connection between the cogitans and the extensa. In the sixth meditation Descartes writes:

(…) there is no doubt that everything that I am taught by nature contains some truth. For if nature is considered in its general aspect, then I understand by the term nothing other than God himself, or the ordered system of created things established by God. And by my own nature in particular I understand nothing other than the totality of things bestowed on me by God.17

In this passage Descartes says that nature in general, in which we must include the thinking and extended substances (for they are created things that are also known by Descartes as belonging to the totality of things), comprises of an ordered system that he is inclined to think of as nothing other than God himself. Now, it goes without argumentation, for otherwise we would not be speaking about God, that God is a self-subsistent entity, a substance in its own right. Descartes’ metaphysical scheme thus does not merely exist of two distinct substances, but includes a third substance in or through which the unity of reality as such is established and on which the other substances depend.18 If we take this into account, we could argue that there is no fundamental bifurcated reality in Descartes’ thought, because he introduces a third substance into his metaphysical scheme that unites the metaphysical realm into an ordered totality. Following this thread, on the highest level, one might say, reality exists of one domain and not two. God is the bridge-principle that connects the two different domains with each other and abrogates the strict division that was made between them. Therefore, Descartes is not only a dualist, but also a monist! Furthermore, Descartes equates, in this passage, God with Nature, which makes God part of natural philosophy (and not merely religion). In doing so, Descartes cleared the way for Spinoza, who indeed argued that there can only be one substance 19 and that this substance is Deus sive Natura (God or Nature). 20

Because God, as a bridge-principle, connects the two domains and we can speak of one single domain on the highest level of reality, the ‘causal closure argument’ against Descartes loses some of its strength. Important for this argument is the claim that each domain has to be causally closed in itself, without the interference of another. Because the two domains of res cogitans and res extensa were (and often still are) thought of as two radical distinct domains that nevertheless mutually interfered with each other, this was considered to be a violation of the causal closure principle. But when we have only one domain, it is perfectly possible that the totality is causally closed (although we might not understand exactly how) and then there is no necessary violation of the causal closure principle in Descartes’ philosophy. Therefore, although Descartes’ answers to the question of how the interaction between res cogitans and res extensa exactly takes place might be unsatisfactory for some and there may be an explanatory gap in Descartes’ writings regarding this point,221 the argument that his metaphysical scheme violates the principle of causal closure per se is not correct.

Nor is it correct to say that Descartes claims that there are mental entities that interfere with the course of action of physical entities that are determined by the Laws of Nature, which would be enough to dismiss Descartes’ philosophy on the more specific argument of the causal closure of the physical domain. We can see that Descartes subscribes to the determination of all physical bodies according to the Laws of Nature (which are, from his perspective, established by God)22 in his The World, as he explicitly says here that “intelligences, or the rational souls…will not disrupt in any way the ordinary course of nature [i.e. Laws of Nature].”23 According to Descartes, these different entities function in a harmonious totality and there is no contradiction in assuming both mental substances and physical substances in one and the same reality.


One may object that Descartes is only able to establish the relation and explain the interaction between res cogitans and res extensa through God and that the assumption of a God-principle has no epistemic value and that it is an unwarranted supposition in science and philosophy which has to be rejected. On account of this critique, there is no place for God in the explanation of reality in contemporary philosophy and sciences. Since there is a widespread consensus that this assumption is indeed no longer acceptable today, the solution that Descartes provides for the interaction between mind and body is no longer accepted and this part of his philosophy is rejected, while the fundamental distinction and the problem of interaction is maintained. But such a partial appropriation of Descartes’ philosophy is problematic, because Descartes was only able to argue for the fundamental nature of the distinction between res cogitans and res extensa through his notion of God24  in the first place and then he established the interaction between these two on account of the same principle. In the first two meditations of his Meditations, Descartes uncovers, through his method of radical doubt, the res cogitans (the thinking subject).25 From the res cogitans he then argued, in the third and fifth meditation, for the necessity of the existence of God. It is only in the sixth meditation that he is able to establish, through the existence of God, that the res extensa – which he did already distinguish from the res cogitans, but could not validate as a substance in its own right, i.e. as having a reality apart from and independent of the thinking subject (up till then the res extensa could have been, for example, dreammatter produced by the thinking substance) – has an objective reality apart from the subject and is a substance in its own right.26 Following the line of argument that is expounded in the Meditations, it is only at the end and by appealing to the God-principle that the substantial distinction between res cogitans and the res extensa can be made and that the substance dualism is born (but also bridged). Since this is done by appealing to the God-principle, God is not only a bridge-principle through which the substance dualism is bridged, but also the principle that allows for the substance dualism to be made by Descartes. Therefore, if one rejects the God-principle as a solution for the mind-body problem, Descartes surely does not have a solution to the mind-body problem, but there is also no reason to argue for a substantial distinction in the first place and assume that there is an unbridgeable gab between the two substances. Rejecting God as a bridge-principle, but maintaining the distinction that is argued for by way of the same principle is like digging a river (which would then be a canal, but I like to play with the metaphor) and then complain that there is no path to get from one side to the other; an avoidable and self-inflicted aporia. Sadly, this is the dialectic that is widely maintained in contemporary philosophy and the ‘Cartesian’ substance dualism is still the big obstacle for contemporary philosophers and scientists. My argument is, however, that, when you appeal to Descartes’ philosophy (and this is still costumery in contemporary literature), you should either reject the God-principle in total, by which there is no unbridgeable dualism, or accept the God principle and have a bridge to bridge the dualism with.

Conceptual Problems in Contemporary Philosophy.

If we still maintain or reintroduce the radical distinction between mind and body without a God-principle, then the nature of this distinction has to be argued for on different grounds. Descartes had a clear conception of the two substances involved and of what the distinction consisted of. This he had established, according to him, with absolute certainty and clarity through God. But what do these substances consist of when we cannot appeal to the reasoning that Descartes used? What do we talk about when we talk about the physical and what do we talk about when we talk about the mental? On what grounds do we maintain the substantial distinction? Here we come to the problem that hunts contemporary philosophy according to my analysis. Because a radical distinction is uncritically maintained and this distinction leads – due to the lack of a bridge principle and the firm confidence in the principle of the causal closure of the physical domain – to an aporia, the idea is that one side of the distinction, in casu the mental, must actually be reduced to the other, in casu the physical, or be eliminated from the picture altogether. For such a reduction or elimination to work, there has to be a clear-cut conception of what the physical is in order to identify the physical as physical and demonstrate that what seems to be non-physical (determined on account of the same definition of the physical) is actually grounded in the physical or to disqualify non-physical notions as unfortunate delusions.27 Without such a clear-cut conception of what the physical exactly is, Physicalist efforts are premature and forced. But it is such a clear-cut conception of what the physical exactly is that is missing and, subsequently, there is no solid criterion to distinguish the (apparent) mind from the matter with.


With respect to definitions in the debate of the mind-body problem, Descartes’ distinction between res cogitans and res extensa (phrased in Latin) is often, without bothering about the notion of God, simply conceived of as the distinction between mind and body (phrased in English) – as if we were speaking about the same thing all along. The claim is then that what is entailed by these categories is self-evident and needs no further clarification.28 This is, however, not the case. Today we are, for example, inclined to think of the physical as solid, tangible matter in the form of particles that collide and interact with each other. Descartes, however, included in his conception of the extensa the depiction of a geometrical figure in the imagination, which we would definitely consider as mental. In line therewith, Barbara Montero argues that, throughout time, very different conceptions of the ‘physical’ and the ‘mental’ have been maintained, that some properties which are sometimes attributed to the ‘mental’ are at other times attributed to the ‘physical’ and that there is not one single definition that designates the common nature of the different conceptions that have been maintained of the ‘physical’.29 This especially poses a problem for those who adhere to the thesis of the causal closure of the physical domain at this point, because what is it exactly that is closed here? Since there is no clarity of the concept, there is no ground to problematize the reality of what is called the mental and to ban it from the (scientific) worldview on account thereof. This does not mean, however, that the causal closure principle of the physical domain has to be thrown out of the window per se, because, if we cannot define exactly what the physical world consists of, there is no reason to assume that what once was or is conceived of as ‘mental’ will never be explainable in ‘physical’ terms, but we simply do not know what that would imply at this point. Now, I fully agree with the analysis of Montero and want to claim that the persistent need to (radically) oppose the mental to the physical is a remnant of the partial (dare I say selective?) appropriation of Descartes’ philosophy. The radical distinction is taken as self-evident, while the fluidity of these concepts and the meaning shift that takes place in the use of these concepts (which problematizes the radical nature of the distinction itself) is neglected.

One could, of course, claim that the meaning shift that takes place in the conception of the ‘physical’ is due to the advancement of the natural sciences, which obtain a more and more accurate view on what the nature of the ‘physical’ is and correct the misconceptions that were maintained in earlier stages. In that case, we only have to look at the current conception of the ‘physical’ as it is defined by theoretical physicists today, in order to have the most accurate conception of what the ‘physical’ entails.30 But besides my reservations about the thought that the natural sciences develop themselves in a linear movement without discontinuities,31there is no general consensus (between e.g. quantum mechanics and relativity theory theorists) on what the ‘physical’ is today and, as Montero points out, it is not clear what the theories of the future, which seem to go in a different direction again, might designate as ‘physical’.32 According to Montero, this future conception of the ‘physical’ might as well turn out to include ‘mental entities’. Besides the point that Montero makes, the claim that the ‘theoretical physicist’ decides on what the ‘physical’ is merely shifts the question of what the ‘physical’ is to the question of who the ‘theoretical physicist’ is. If there are different scientists who seem to qualify for this position, but they do maintain different conceptions of the ‘physical’, who is the right ‘theoretical physicist’? That would, of course, be the one who describes the ‘physical’ accurately. In order to decide on that, we need to know what the ‘physical’ is. Here we obviously have a problem of criterion again.

In any case, it seems to be hard to pin down what exactly the ‘physical’ is and subsequently also what the ‘mental’ is that is opposed thereto, if we cannot make use of the reasoning that Descartes had put forth. This uncertainty about the meaning of these concepts, to my opinion, does not entail that we should not make a distinction between the mental and the physical at all. But it does entail that the presupposed and radical division between these two is not inevitable. Actually, the boundary that divides them from each other is rather fluid and can be crossed, if only we do not fear the water and learn how to swim. But whatever path one will find, nor the acceptation of Descartes’ philosophy, nor the rejection thereof gives rise to an insurmountable mind-body problem; it is only a strange amalgam of rejection and acceptation that gives rise to this problem.