The Metaphysics of Morals by Immanuel Kant Chapter 3 Page 39

that is, of a faculty distinct from mere desire (namely, a faculty of determining itself to action as an intelligence, in other words, by laws of reason independently on natural instincts).

Now where determination according to laws of nature ceases, there all explanation ceases also, and nothing remains but defence, i.e., the removal of the objections of those who pretend to have seen deeper into the nature of things, and thereupon boldly declare freedom impossible. We can only point out to them that the supposed contradiction that they have discovered in it arises only from this, that in order to be able to apply the law of nature to human actions, they must necessarily consider man as an appearance: then when we demand of them that they should also think of him qua intelligence as a thing in itself, they still