1. A “merely probable” argument for the existence of material things (61R-62L).

This argument rests on a distinction between two faculties of the mind: the intellect and the imagination.¹ The example of the triangle and the chiliagon illustrates the distinction. You can understand that triangles have three sides and chiliagons have one thousand sides. But while you can have a mental image of a triangle that looks different from a square, you cannot have a mental image of a chiliagon that looks different from a figure with one thousand and one sides. Consequently, understanding a geometrical figure must be different than imagining it.

Descartes reasons that since imagination involves the body the existence of the imagination strongly suggests, without proving, that bodies exist.

I am not confident that I follow his reasoning when he says that the imagination involves the body. Still, it strikes me as plausible. If you think that mental images are based on things you have seen and if you think that seeing involves having eyes that receive light reflected off of objects, for example, you might well think that imagination depends on bodies.

2. A review of the senses

Descartes reviews what he thinks he knows based on the senses (62R-63L) and reasons for doubting that the senses are always reliable (63R).

3. The distinction between mind and body

Descartes knows that he is a thinking thing, distinct from his body (64L). We will pay a lot of attention to this.

(1) I know that all the things that I clearly and distinctly understand can be made by God such as I understand them. (2) For this reason, my ability clearly and distinctly to understand one thing without another suffices to make me certain that the one thing is different from the other, since they can be separated from each other, at least by God. The question as to the sort of power that might effect such a separation is not relevant to their being thought to be different. (3) For this reason, from the fact that I know that I exist, and that at the same time I judge that obviously nothing else belongs to my nature or essence except that I am a thinking thing, I rightly conclude that my essence consists entirely in my being a thinking thing. (4) And although perhaps (or rather, as I shall soon say, assuredly) I have a body that is very closely joined to me, (5) nevertheless, because on the one hand I have a clear and distinct idea of myself, insofar as I am merely a thinking thing and not an extended thing, and because on the other hand I have a distinct idea of a body insofar as it is merely an extended thing and not a thinking thing, it is certain that I am really distinct from my body, and can exist without it. (Descartes [1641] 2019, 64L) (numbers added)

4. The nature of the mind

Descartes argues that the mind is an intellectual substance (64).

The senses and imagination are part of the mind, but they are not essential to it in the way that reasoning (“intellection”) is. The idea is that he can imagine thinking without having sensations or mental images. Consequently, those things are not essential to his existence as an intellectual substance.

5. External objects exist

I have sensations and imaginations. But while I can imagine things at will, I have no direct control over my sensory experiences. So they must have a source outside of me. That source could be external objects, God, or some other higher being. Since God is not a deceiver, we can conclude that the source is what it seems to be: external objects (64R).

6. The relationship between mind and body

The mind is “commingled” with the body. The mind feels pain when the body is damaged. Another way of understanding the relationship is that they are entirely separate, much as a sailor is different from the ship he is sailing in. But Descartes does not think body and mind are that different. A sailor can only see when the ship is damaged; he would not feel it (65L).

The body usually conveys accurate information to the mind, but it does not always do so. For example, someone suffering from dropsy, a condition in which the body retains excessive fluid, will feel thirsty. Since thirst is usually a sign that we need to drink, this would lead us to do the wrong thing (66R).

Why doesn’t God prevent errors like this? This question leads Descartes to make four observations about the relationship between the mind and body (67).

1. Mind and body are distinct because bodies are divisible but the mind is an indivisible unit (67L). This is another important passage for our discussion of the difference between mind and body.

2. The brain is the part of the body that most immediately effects the mind (67L).

3. A mechanistic theory of how sensations work. A pain in the foot pulls the nerves in the foot which, in turn, “pull on the inner parts of the brain to which they extend, and produce a certain motion in them” which, in turn, produces “a sensation of pain” in the brain (67R).

4. God set up the body to generally convey corect information. When the brain feels pain, the mind decides to move the foot. While there can be errors, God gave us the ability to figure them out using our other senses and memory (67R-68L). For example, if I have seen other cases of dropsy, I will know not to drink when I am suffering from it even if I feel thirsty.