Earlier thinkers such as Plato, Aristotle or Thomas of Aquinas have thought about the metaphysical status of the soul and the relationship between mind and body, but it was the French philosopher René Descartes who introduced a unique understanding of dualism into early modern philosophy. This article aims to show that Descartes’ dualism holds the substantial claim that body and mind are two distinct substances.

At the beginning of his Discourse on the Method, Descartes expresses his disappointment with the readings of philosophy as there is „not a single matter which is not still in dispute”1. Thus, his leading question in the quest for knowledge is: What can I know for certain?2 What is it that I cannot doubt to exist whether I might be awake, asleep or dreaming? Is it the senses (body), reasoning (mind) or my existence even? His skeptical method of doubt asks for the one certain and unshakable reason to believe something that is indubitable. And Descartes claims that the only thing that he conceives „clearly and distinctly”3 is his thinking. Even if all his thinking might be mistaken, he can be sure of the fact that he is in fact thinking. This is his first item of knowledge („I am, I exist”) which is an a priori truth gained by mental scrutiny and which holds true in this and every possible world. But what does Descartes mean by “I”? In his second Meditation he defines “I” as „a thing that thinks” or „a thinking thing” (res cogitans). In Descartes’ line of thinking, man is made up of mind (indubitable) and body (dubitable). Human being is both a mental and a physical being but with different essential properties: The essential property of the mind, which is the necessary reason to see the truth, is that it is a res cogitans. Descartes infers that it is immaterial in virtue of it being a distinct substance from the body. It is indivisible and private because only the person in whose body the mind resides experiences the workings of his own mind. Thus, there is a subjective, first-person access to the mind. The body however is a matter that is spatially extended (res extensa). It „has a determinable shape and a definable location”4 and can be perceived by the senses. Given its material nature it is changeable, meaning that its shape is flexible and can therefore change due to external conditions or extraneous causes. This corporeal nature can be grasped from the outside, thus there is a public access to it. But how are these two distinct substances connected? Descartes purports in The Passions of the Soul that even though the mind is „joined to the whole body”, it is the pineal gland5 in the brain where the soul (Descartes’ synonym for mind, Discourse 4, 2) „exercises its functions”.6

The interaction of body and mind in the dualistic being of humans still poses the central mystery of human life and a conundrum of our existence: What if our mind is incapable of comprehending itself?

1 René Descartes: Discourse on the Method. 1, 12.

2 René Descartes: Meditationes de prima philosophia. II.

3 René Descartes: Discourse on the Method. 1, 2.

4 René Descartes: Meditationes de prima philosophia. II.

5 Descartes is wondering how the two separate parts of body and soul are interacting. He is looking for a tiny part through which the mind is affected by the brain. How can the soul bring about actions in the body? It is through the pineal gland that is located in the human brain (diencephalon) according to Descartes. The pineal gland is in this respect a kind of organ of the soul that exerts influence on the body. It isn’t the human brain as such, but the pineal gland that Descartes regards as the principal seat of the soul.

6 René Descartes: The Passions of the Soul. 31: „Qu’il y a une petite glande dans le cerveau en laquelle l’âme exerce ses fonctions plus particulièrement que dans les autres parties.”