In the Preface of the Groundwork of the Metaphysics of Morals Kant says he believes that moral philosophy must be devoid of inputs from human nature (“anthropology”):
[Starting at about 4:389]
“Since my aim here is directed properly to moral philosophy, I limit the question proposed only to this: is it not thought to be of the utmost necessity to work out for once a pure moral philosophy, completely cleansed of everything that may be only empirical and that belongs to anthropology.”
In the very next sentence he explains why he believes this:
“For, that there must be such a philosophy is clear of itself from the common idea of duty and of moral laws.”
He doesn’t dwell on this any further. He doesn’t question whether moral laws actually exist and are waiting to be discovered. Instead, in the very next sentence he is off talking about moral laws as if they’re as real and absolute as the laws of physics.
“Everyone must grant that a law, if it is to hold morally, that is, as a ground of an obligation, must carry with it absolute necessity; that, for example, the command “thou shalt not lie” does not hold only for human beings, as if other rational beings did not have to heed it, and so with all other moral laws properly so called; that, therefore, the ground of obligation here must not be sought in the nature of the human being or in the circumstances of the world in which he is placed, but a priori simply in concepts of pure reason…”
In the above chain of argumentation I read two premises that Kant thinks justify the rest of his moral project:
1) Universal moral laws exist. Trust me.
2) Obviously we can't ground universal moral laws in something as variable as human nature (so now I'm going to show you how to derive the categorical imperative by pure practical [a priori] reason).
Are these Two (2) Premises Potential Flaws in Kant's Moral Theory?
To be clear, I'm not questioning any of Kant's moral argumentation that follows the quotes I've offered here (remember, these are from his Preface). I'm focusing on the premises that set up his moral theory. Are they justified? Does Kant do a good enough job of making his case for them?
PS - For the record, I don't think either of these premises are justified but I am well aware that many do. Perhaps you are one of them. Perhaps you feel I've misinterpreted something in the above quotes, or maybe you know of other passages in Kant's work that better explain these premises or offer different premises to justify his insistence that morality can only be grounded in pure practical reason. I look forward to hearing from you.
Kant, Immanuel (2008). Practical Philosophy. In Mary Gregor (Ed. and Trans.), Groundwork of the Metaphysics of Morals by Immanuel Kant. Cambridge University Press. New York, NY. (Original work published 1785).