For Descartes, reality consists of two fundamental substances: the mental, and the physical.
The mental substance is distinct from the physical substance in that it is not bound by the deterministic, natural laws. This, in turn, implies free-will.
However, if the mind is to control the body, and if the body is, in fact, governed by the laws of a deterministic universe, isn't, then, the mind incapable of free-will?
By definition, the notion of free-will is the denial that determinism applies to the actions of human beings. Even if we suppose that the mind is not subject to determinism, if the body is then is the mind not also restricted by the natural laws?