In Chapter 4, I show how Aristotle appeals to his demonstrative heuristic to show that Democritus is able to provide a formal, but not adequate, explanation of both the soul’s ability to cause the body to move and its ability to think. This is because he thinks Democritus lacks an explanation of our ability to have a stable form of cognition from which deliberated actions proceed. What Aristotle gains from this criticism is the idea that the soul’s production of local motion, even if it involves mechanistic causes within the body, must also involve (at least in the case of humans) a free decision to move or to not move. The result of this criticism implies that the explanation of animal behaviour cannot be reduced to a deterministic series of causal interactions, and hence, that Democritus’ definition of the soul, which implies such a determinist account, is not scientific. The criticism of Democritus helps to show, indirectly, that the soul operates in an intentional way as both a final and efficient cause in the origination of the motion of animals, and thus lays further groundwork for the Efficient-Final Causal Thesis.