But a substance can be understood to exist alone without requiring any other creature to exist. That is, its existence is not dependent upon the existence of minds or other bodies; and, a stone can exist without being any particular size or shape.
This indicates for Descartes that God, if he chose, could create a world constituted by this stone all by itself, showing further that it is a substance “really distinct” from everything else except God.
For this reason, a brief look at how final causes were supposed to work is in order.
Descartes understood all scholastics to maintain that everything was thought to have a final cause that is the ultimate end or goal for the sake of which the rest of the organism was organized.
Accordingly, any dispositions a swallow might have, such as the disposition for making nests, would then also be explained by means of this ultimate goal of being a swallow; that is, swallows are disposed for making nests for the sake of being a swallow species of substance.
This explanatory scheme was also thought to work for plants and inanimate natural objects.
A criticism of the traditional employment of substantial forms and their concomitant final causes in physics is found in the But what makes it especially clear that my idea of gravity was taken largely from the idea I had of the mind is the fact that I thought that gravity carried bodies toward the centre of the earth as if it had some knowledge of the centre within itself (AT VII 442: CSM II 298).
On this pre-Newtonian account, a characteristic goal of all bodies was to reach its proper place, namely, the center of the earth.
One of the deepest and most lasting legacies of Descartes’ philosophy is his thesis that mind and body are really distinct—a thesis now called "mind-body dualism." He reaches this conclusion by arguing that the nature of the mind (that is, a thinking, non-extended thing) is completely different from that of the body (that is, an extended, non-thinking thing), and therefore it is possible for one to exist without the other.
This argument gives rise to the famous problem of mind-body causal interaction still debated today: how can the mind cause some of our bodily limbs to move (for example, raising one's hand to ask a question), and how can the body’s sense organs cause sensations in the mind when their natures are completely different?