When it comes to ethics, Immanuel Kant is most well known for his formulation of the "categorical imperative." In short, this rule states that we should act in such a way that our actions would be in accordance with a principle that would suffice as a universal law. Kant arrives at this rule after a lengthy argument, but the purposes of this post really is limited to his distinction between the "form" and "matter" of a universal law.
The "form" of a universal law is generally what it sounds like, the basic form of the law that makes it universal by nature. The "matter" is the specific object of that law. "Forms," according to Kant, are arrived at through pure practical reason. "Matters," on the other hand, are discerned through the human senses.
From these premises Kant argues that in order for a person's will to be truly "free," it must be determined on the basis of the form of universal law (which he concludes to be the categorical imperative through an argument not relevant here), not the matter. Natural laws, so Kant urges, are only discernable by using the human senses (something Kant calls "appearances"), therefore they are not the subject of pure reason and belong to the category of "matter." If the human will were determined by the operation of natural laws then it would not be truly free (i.e., the result of the will would be mechanically determined by the operation of those laws). This allegedly would not be a problem if the will could be determined by universal laws that can be cognized through pure reason without appealing to anything perceived via the senses. By being determined by the form of universal law as opposed to the matter of the natural law (which is only perceived through the senses), the will avoids the danger of losing its freedom. Therefore, a will that is determined by the form of a universal law is free whereas one determined by the matter is not.
Kant’s Critique of Practical Reason claims to demonstrate that just such a universal law is practical in the form of the categorical imperative. Therefore, a free will is one that is determined by the categorical imperative (which Kant says is synonymous with the moral law). Kant believes we must assume the moral law exists. But if so, this carries with it the entailment that free will also exists. Because we must assume the moral law exists, we similarly must assume the existence of freedom.
In many respects I agree with Kant. For example, as I have argued elsewhere I agree that we must assume the existence of free will before any rational discussion can begin. While I also have several issues with Kant's argument, the point of this post is to focus on only one subpart. Specifically, in working his way toward the conclusion of free will, Kant claims that the determining basis of anything that is determined in accordance with the natural law must lie amongst things that are objects of the senses. However, Kant appears to be smuggling in two mutually exclusive perspectives on the laws of nature in order to support his argument. The laws of nature may be viewed either as descriptive, meaning the statement of a “law” is merely a description of what we have generally observed to happen in the past with no guarantee that this will continue to hold true in the future, or prescriptive, meaning they are fixed rules like the laws of logic and will continue to operate into the future regardless of whether we have ever perceived them to operate in the past. In order for the laws of nature to mechanically determine a will it appears to me that they must be prescriptive, meaning they operate whether we perceive them or not. But in order for the natural law to belong to the category of “appearances” (i.e., objects of the senses, a necessary premise in Kant’s argument for the freedom of a will determined by universal legislation) it must be only descriptive. In one breath Kant wants the laws to be prescriptive so they can determine a will but in the next he needs them to be descriptive so that they are only appearances and cannot be the subject of pure theoretical reason.
It seems to me that Immanuel Kant attempts to have his cake and eat it too.