He deliberately chooses to disbelieve all that can be doubted. The foundation for all that follows is the absolute certainty of his own existence in the midst of doubting. A mind is entirely an interiority, known only through introspection. It cannot be directly known from the outside. It is not a body, not extended.
Bodies are entirely exteriorities, known only from the outside.
Sense experience is conceived of as representative appearances within the mind of things outside the mind, somehow produced by those things through the medium of sense organs. There is good reason to believe the mind or soul is capable of surviving the death of the body. The living organism is thus a composite of form and matter, soul and body. There is reason to doubt that the mind survives the death of the body, though Aristotle is not entirely clear on this point. That is, a person is not one thing and his mind another thing.