The Metaphysics of Morals by Immanuel Kant Chapter 2 Page 18

moral instruction, so as to produce pure moral dispositions, and to engraft them on men's minds to the promotion of the greatest possible good in the world.

But in order that in this study we may not merely advance by the natural steps from the common moral judgement (in this case very worthy of respect) to the philosophical, as has been already done, but also from a popular philosophy, which goes no further than it can reach by groping with the help of examples, to metaphysic (which does allow itself to be checked by anything empirical and, as it must measure the whole extent of this kind of rational knowledge, goes as far as ideal conceptions, where even examples fail us), we must follow and clearly describe the practical faculty of reason, from the general rules of its determination to the point where the notion of duty springs from it.