In his Principles of Philosophy Descartes claims that the freedom of the will is one of the “common notions that are innate within us” and therefore does not need to be defended with argument. But as obvious as the notion of freedom of the will is the notion that God has pre-ordained all things, including our own actions. Descartes offers only our imperfect epistemic state as an attempt to reconcile these two incompatible truths. We are not in a position to understand how the two are compatible, but instead must simply assent to both propositions as known (CSM I 205-6). In his response to Hobbes' objections to the Meditations Descartes claims that the freedom of the will is “very evident by the natural light” (CSM II 133-4).
So Descartes will likely not be a place to find a full answer to the modern problem of free will and determinism, since he simply asserts compatibilism without arguing for it. But this idea that God causes everything is a form of determinism and it fits uncomfortably with the idea that we are responsible for our errors in judgment. So his versionof free will is in fact a version of compatibilism. Descartes also, despite his dualism, sought to provide an account of the relationship between the free mind and the deterministic world, of which the body is a part.
Descartes seeks to illuminate how error arises, and in what sense our will can be considered to blame. In the Meditations Descartes locates the cause of error in the will. He doesn't spell it out, but his logic would hold whether we make an error in moral judgment as well as when we make a more run of the mill error. Errors have two concurrent causes: the intellect and the will. Intellect, however, cannot contain error because it merely presents ideas in perception. The will makes judgments about those ideas, and it is these judgments wherein error lies. But this is not because the will is imperfect; the will itself is by its nature completely unfettered and unlimited and this limitlessness of the will is a perfection. The problem, then, cannot be in the nature of the will (CSM II 39-40).
The problem is that the will has a wider scope than the intellect, which is not limitless. Instead of judging only when the intellect possesses clear and distinct ideas, the will sometimes makes judgments on mere appearances, confused ideas, or probable conjectures. The conclusion here is that if we refrain from making judgments except when we have clear and distinct ideas we will not err (CSM II 40-1).
We should not, according to Descartes, complain about this state of affairs or blame God for mismatching our will and intellect, by the way. The intellect, as a finite and created thing, could not be otherwise than it is. Neither could the will, since it is an indivisible thing, have anything taken away from it or added to it. Nor can we complain that acts of judgment that lead to error occur with God’s concurrence. God cannot be held responsible for the misuse of the freedom of the will when it makes judgments without the sufficiently clear and distinct ideas. God could have made men with will and intellect with identical scope, and even though men would be more perfect, it was not God’s design, and so should not be questioned (CSM II 42-3).
Descartes' insistence that God must be absolved of responsibility for the inherent vice of his creation is essentially an argument from ignorance: we don't know why God did what he did, but since he's God, we have to trust that it's for the best. But Descartes does explore more fully the interaction between the soul and the body in his Passions of the Soul. In doing so, he does attempt to provide a mechanism for the role of the will in human action.
Even though the soul is joined to the whole body, it “exercises its functions” in the pineal gland “more particularly” than in the other parts of the body. Descartes comes to this conclusion because all the other parts of the brain are double. The brain is left/right symmetric, and the sensations we receive have to come together in one place so that the soul sees only one image (CSM I 340). The soul has its seat in the pineal gland, and “radiates through the rest of the body by means of the animal spirits, nerves, and even the blood, which can take on the impressions of the spirits and carry them through the arteries to all the limbs.” The pineal gland is situated so as to be able to be moved by both the body and the soul. The mechanism of our body is constructed to allow the movement in the pineal gland caused by the soul to drive “the surrounding spirits toward the pores of the brain, which direct them through the nerves to the muscles; and in this way the gland makes the spirits move the limbs” (CSM I 341). One obvious flaw with Descartes' theory, of course, is that he does not provide us with any explanation for how soul-stuff and body-stuff interact. But the problems of dualism have been discussed elsewhere on this blog so I won't belabor the point.
Not all functions of the body can be immediately controlled by the soul, according to Descartes. Autonomic processes and automatic reactions, as well as the passions, can be affected by the will only mediately. For the soul to affect the passions, it must engage reason in addition to the will. The reason the will lacks full power to change the passions is that the passions are to a large extent dependent on mechanical processes that have physical effects (CSM I 344-5). Descartes claims that the conflicts that had traditionally attributed to a conflict between the “sensitive soul” and the “rational soul” (like in Plato's model of the soul) are more accurately described as conflicts between the mind and the body. The soul is a unity, with no diversity or parts. The body is the locus for all the nonrational impulses, as the soul is for the rational capacities. There are two sorts of movement produced in the pineal gland by spirits: (1) sense data, which have no influence on the will, and (2) passions, which do. Conflict in the soul is between the will and the passions (345-7).
The strength of the soul is measured by whether the agent can make the correct determinate judgments regarding moral questions in the face of the influence of the passions. Strength of will is insufficient for proper action without accurate judgment of truth and falsity. Despite the seeming power of the passions, there is no soul too weak to master the passions and act according to reason given sufficient training and guidance (347-8).
So it can be seen that Descartes is attempting to incorporate the
idea of material processes influencing the activities of the soul into
his account of human failures, including moral failures. But it is not the intellect that is at fault. Instead, the fault lies in the will, which appears to operate on the border between the soul and the body. In order to act and judge rightly and in accord with the information available to the intellect, an agent must possess sufficient strength of will to overcome the passions that affect it and might prompt it to act without being properly informed by the intellect (hence the mismatched scope of the two faculties referenced above). Descartes' view hold that the rational and emotional parts of ourselves are not only distinct, but truly at odds. The emotions have to be suppressed and battled if we are to act and judge rightly. This view of the relation between reason and emotion is in stark contrast with Hume's claim that reason "is, and ought only to be, a slave to the passions," as well as the findings of modern neuroscience (see Damasio's Descartes' Error).