Abstract

On the premise that ‘morality’ refers to a system of rules governed by a material categorical imperative, I argue that the sources thesis of legal positivism (and, consequently, its separation thesis) is untenable. This is because it portrays legal obligations as hypothetical imperatives, which they cannot be if a material categorical imperative exists. Legal systems lay down obligation-asserting rules; but any rules are necessarily invalid if they require behaviour contrary to a material categorical imperative. Because the sources thesis attests otherwise, legal positivism presupposes, minimally, that a material categorical imperative cannot be known to exist. I argue, however, that human agents cannot coherently question that a material categorical imperative exists. By definition, they necessarily adopt hypothetical imperatives. But understanding the concept of a hypothetical imperative requires the concept of a categorical imperative to be understood, and understanding having the concept of a categorical imperative postulates that there can be only one categorical imperative, that it exists, and that it is Alan Gewirth’s Principle of Generic Consistency (PGC). This argument only fails if the concept of a categorical imperative is incoherent. In all of this, I follow Kant, except about the material content of the categorical imperative and the nature of free will.