Descartes’s notion of divisibility can be confusing. However, if we break it into smaller parts (something he likes to talk about), we can begin to understand how it informs his arguments about the mind.
Descartes claimed that something is divisible if it can be broken down into similar parts. For example, he claimed the body was divisible. Let’s think more about this. If one was to break down the human body into smaller parts, would all of the parts be exactly the same? Definitely not. However, Descartes argued that all of the parts would be made out of the same substance.
On the other hand, Descartes argued that this concept of divisibility does not apply to the mind. This is because he cannot conceive of the mind having parts the same way that the body does. He also stressed that the body is material, or physical, while the mind is immaterial.
What does Descartes mean by substance? Descartes theorized that a substance is something that exists independently because it is created by God. The three substances according to Descartes are God, the mind, and the body.
The simplified argument for mind-body dualism is as follows:
Descartes believed that whatever he could clearly and distinctly perceive was created by God. And because God is perfect, what is clearly perceived is trustworthy. Descartes could clearly and distinctly perceive that the body has parts. He could also clearly and distinctly perceive that the mind does not have parts. If the mind and the body were the same, then it would be a thing that both has parts and does not have parts. This would be impossible. Therefore, the mind exists independently from the body.