Kant's indictment is based on two premises. The first premise is that, in order to be good, there must be a constraint upon the will to act in a specific fashion, whether or not it happens to be what one desires at that moment. The fact of our desire cannot determine what we are supposed to do. The second premise is that this constraint—call it "duty"—must not only determine the will; we must be able to freely choose it. Otherwise we're talking about coercion, not morality.
Neither of these premises seems self-evident, but the second one at least accords with our intuitions about morality. If you're forced to pursue good ends—a la the character Alex in A Clockwork Orange—then you are not freely choosing the good action and so you cannot be considered good. This is not to say that the goodness of an action reduces to the intention that brings it about (though Kant seems to think so), just that intention and freedom cannot be left out of the balance when we are considering morality. Of course not all accounts of the good are premised upon Kant's notion of radical freedom, but they all seem to include some minimal account of agency. That is, a good action is premised upon some sort of activity of the person who is considered good or acting in accordance with the good. This is true of Aristotle as much as it's true of Kant.
The first premise—that the goodness of an action must be considered in abstraction from the fact of desire—seems to have less intuitive evidence in favor of it. In my opinion this premise is more obviously historically conditioned, though it is not for that reason any more objectionable than the second. The first premise rests on a radical distinction between fact and value. Minimally it is grounded in the idea that just because something is the case, it does not follow that it ought to be the case. Maximally it means that the ground of moral judgment is absolutely independent of sense experience.
The minimal version of this premise seems to be an aspect of human experience itself which we might call transcendence. Humans are not merely aware of what presents itself immediately to the senses. They are also aware of the fact that they are being appeared to this way. There is what we immediately perceive or desire, but there is also what we imagine or conceive to be beyond that which is sensuously given. Humans populate this supersensible realm with spirits, powers, forces, and mathematical laws of nature, and what we immediately perceive becomes the expression of that underlying, truer reality. Yet transcendence does not occur for the first time with the rise of art, religion, and science. It's a feature of perception itself. I don't just perceive redness, sweetness, hardness, etc. I perceive that these are properties that belong to a piece of fruit and that could belong, jointly or separately, to other objects in the world. Already in basic perception, I am beyond the immediate sense appearances and am aware of general things like properties and objects. This, too, is a feature of transcendence. So is the fact that I can contemplate the past and the future and think about what my life means. Part of living that life means I have to drink water, because I'm an animal. But it also might mean ignoring that desire because the recognition I'll acquire by crossing the desert is more important to me. Neither nature within me nor without me is merely given to me. I am aware of and participate in a world of norms and universals that I use to judge and make decisions about how I will act in the material world. What I desire at the moment might be absolutely irrelevant to what I ought to do right now. This minimal notion of the ought is present in virtually all philosophy and seems to be a basic feature of human experience.
Yet Kant seems to believe the stronger, maximal version of this: moral judgment occupies a sphere absolutely independent of sense experience. More than that: the will operates in a realm absolutely free of any kind of natural causation. This is the ground of the will's absolute freedom, but it also the source of the prohibition on basing any maxim of action on anything sensuously given. I can base my actions on any principle I want, but the good action is the one which is based on a principle given by pure reason itself. So whatever the moral law is, it's not good because it comes from a good object, be it nature, God, whatever. It's the other way around. Whatever the good object is, it attains its goodness by its action according with a law given by reason itself. It's a radical, profound statement. Nothing in this world or any world is intrinsically good or just. Every human political institution must answer to reason itself. We decide what is right and what is wrong, regardless of what has come before or what exists. It's a powerful, compelling modernist idea, the force of which was experienced in France in the 18th century and which is being experienced in Libya right now. In thinking about this assumption, we're doing more than thinking about Kant's moral philosophy. We're thinking about why human action has come to have a power to reorganize the whole of society and the planet. Because the fact/value distinction at work here really is a total opposition. It represents a break with all previous intellectual thought, but more importantly, it represents a real break in the material world with a previous way of living. This idea is really given its full articulation in Kant's philosophy. Why does Kant seem to think this?
The usual way to do this would be to present the argument in the Third Antinomy in the Critique of Pure Reason and from there to move on to the elaboration in the Groundwork of the Metaphysics of Morals. I'm not going to do that because (a) it would take too long and (b) Kant presents his arguments in those works as though they are true a priori and I don't believe they are. As I'll explain later, I think the fact/value distinction has the force of an a prior (there's necessity in it), but it's in fact a historical accomplishment.
The short answer to why Kant believes in a radical fact/value distinction is one word: Newton. Kant believed the natural world was a causally-closed realm governed by the sorts of mathematical laws Newton had articulated in the Principia. Modern science abolishes superstition, but it would also appear to abolish freedom and hence morality and values. The Critique of Pure Reason is an attempt at a rapproachment between knowledge and morality: Newton's laws govern the reality we perceive with the senses, but only the moral law can govern the purely intellectual reality in which the will operates. This is what Kant means when he says he had to limit knowledge to make room for faith. He did not mean faith in God. He meant faith in the possibility of absolutely free action, an idea he proceeds to flesh out in subsequent works. This limitation on knowledge is possible by virtue of Kant's "Copernican" turn. This is the idea that objects must in some sense accord with the way we know things and not the other way around. Kant not only compares himself with Copernicus. He sees the very same pattern of thought all over the modern scientific method:
When Galileo caused balls, the weights of which he had himself previously determined, to roll down an inclined plane; when Torricelli made the air carry a weight which he had calculated beforehand to be equal to that of a definite column of water; or in more recent times, when Stahl changed metal into lime, and lime back into metal, by withdrawing something and then restoring it, a light broke upon all students of nature. They learned that reason has insight only into that which it produces after a plan of its own, and that it must not allow itself to be kept, as it were, in nature's leading-strings, but must itself show the way with principles of judgment based upon fixed laws, constraining nature to give answer to questions of reason's own determining. Accidental observations, made in obedience to no previously thought-out plan, can never be made to yield a necessary law, which alone reason is concerned to discover. Reason, holding in one hand its principles, according to which alone concordant appearances can be admitted as equivalent to laws, and in the other hand the experiment which it has devised in conformity with these principles, must approach nature in order to be taught by it. It must not, however, do so in the character of a pupil who listens to everything that the teacher chooses to say, but of an appointed judge who compels the witnesses to answer questions which he has himself formulated.The idea here—and this applies equally to metaphysics and science—is that understanding is relative to a specific kind of human activity. This contrasts with the pre-Kantian idea which is that understanding is relative to an optimal, "God's eye" perspective in which a representation accords with (or fails to accord with) an object outside of consciousness. Kant noticed that Galileo, Torricelli, and Stahl didn't just look at nature and write down what they saw. Neither did they simply count out what they saw, as though the application of mathematics was somehow peculiar to the modern age. He saw that they designed experiments, and that everything they discovered was due to this original action.
Modern science agrees with Kant. The idea behind general relativity is that spatiotemporal relationships between distinct events are relative to the experimental setup used to determine those relations. (See Geroch, General Relativity from A to B) Most people who look at the Kant/Einstein connection look to Kant's writings on space and time in the Transcendental Aesthetic section of the first Critique. This is an error, since Kant's concepts of space and time are largely Newtonian and conservative. He argues there that transcendental idealism is the view that there are subjective, a priori forms of experience. In other words Kant intellectualizes and subjectivizes what is in fact something that occurs in the real world. He's much closer to the mark in his resolution to the First Antinomy where he argues that whether or not the universe is bounded or limited can only be answered relative to the actual way we go about studying the cosmos. There Kant seems to be saying that knowledge is a special kind of human activity carried out in the real world. And natural science, of course, is human knowing par excellance.
We set up experiments in which we control for certain variables, and by doing so we get nature to tell us the mathematical relationships between those variables. This is why Kant says that reason is "an appointed judge who compels the witnesses to answer questions which he has himself formulated." What the natural philosophers of the 17th century did for our knowledge of nature, Kant suggests we do for our knowledge of metaphysics. So in comparing himself with Copernicus, Galileo, Torricelli, and Stahl, Kant is not reading his own philosophy back into the natural philosophy of the 17th century. What Kant embodies in his philosophical thought is a real movement that had been taking place in physical reality for at least two centuries if not more. It's because of this historical transformation that took place in the way people relate to one another and the natural world that Kant begins philosophizing from a radical fact/value distinction.
What this means is that the fact/value distinction is a presupposition of modern science, but it is a result of it, too. How is that possible? The fact/value distinction is something that human beings bring about through technology and through the construction of experiments that yield cognitions of nature. The cognitions it brings forth are themselves mathematical, objective, and therefore value-neutral, thereby reinforcing the initial assumption. And it is only in this context, I would argue, that a Kant could come along and say that the fact of our desire cannot tell us what we ought to do. Surely something like this idea existed before modernity. That's the minimal version of transcendence I articulated earlier. But Kant's view is premised on a radical fact/value distinction which I think is particular to an age in which nature has come to be understood as a causally closed, mathematical whole.
One might ask at this point: If the fact/value distinction is not a factual distinction but instead a value distinction and one which is historically conditioned, why should we accept it? Doesn't that make it arbitrary?
No. Just because something is historically conditioned, that doesn't make it arbitrary; nor does it mean you could just wave your hand and change it. Perhaps it was necessary that, at some point, some humans would develop their technology to the point to have control of nature sufficient enough to do modern science. From that would follow the fact/value distinction and the Kantian way of understanding morality. That would be a lengthy and difficult argument to make, but it's not out of the realm of possibility. The only point is that, just because something did not always exist but came into being at some point, that does not make it arbitrary.
I mentioned earlier that the fact/value distinction seems to have the force of an priori, even though it seems to be historically conditioned. In order to understand it this way, we need some notion of historical necessity or of a material a priori. This is the idea that something sensuous and particular can have all the authority and all the power to orient us as one of Plato's ideas. Kant's own philosophy rules out such a material a priori. Despite the radicalism of his Copernican turn, Kant appears to believe the fact/value distinction he assumes is a basic feature of the universe. For him, the fact/value distinction is a factual distinction, one we must assume. I'm claiming it's a value distinction or at least one which was brought about through the real activities of real human beings. That strikes me as a more "Kantian" formulation than Kant's own. (Fichte and Hegel seem to have agreed, which is why they took Kant's philosophy in that direction.)
So those are the assumptions of Kant's moral philosophy. They're not without their problems. Though I would argue that the problems are not trivial problems. Like most of the really interesting problems in Kant's philosophy, they reflect contradictions that exist in material reality. Getting rid of them is not a matter to be taken lightly. This is even more evident in Kant's moral philosophy, which I'll talk about in the next post.