But now I must explain how the mere fact that I can clearly and
distinctly understand one substance apart from another is enough to
make me certain that one excludes the other
    The answer is that the notion of a substance is just this - that it
can exist by itself, that is without the aid of any other substance. And
there is no one who has ever perceived two substances by means of
two different concepts without judging that they are really distinct.
    Hence, had I not been looking for greater than ordinary certainty,
I should have been content to have shown in the Second Meditation
that the mind can be understood as a subsisting thing despite the fact
that nothing belonging to the body is attributed to it, and that,
conversely, the body can be understood as a subsisting thing despite
the fact that nothing belonging to the mind is attributed to it. I should
have added nothing more in order to demonstrate that there is a real
distinction between the mind and the body, since we commonly judge
that the order in which things are mutually related in our perception of
them corresponds to the order in which they are related in actual
reality. But one of the exaggerated doubts which I put forward in the
First Meditation went so far as to make it impossible for me to be
certain of this very point (namely whether things do in reality
correspond to our perception of them), so long as I was supposing
myself to be ignorant of the author of my being. And this is why
everything I wrote on the subject of God and truth in the Third, Fourth
and Fifth Meditations contributes to the conclusion that there is a real
distinction between the mind and the body, which I finally established in
the Sixth Meditation. (4th replies)