Paul Guyer

"With these dramatic words, Kant alludes to the two great problems and accomplishments of his philosophical career. On the one hand, he wants to know how we who as creatures are a mere part of nature can discover how all of nature … does and even must work. On the other hand, he wants to display the unconditional value that we have as rational rather than merely natural beings, … and that we are always free to act in accordance with and indeed for the sake of this principle, thus free to realize the unconditional value for which we unlike anything else in nature have the potential."

"It may seem as if Kant was content with such a radically dualistic view of human action, but ultimately he was not. … What Kant is assuming here is that morality is not just a matter of making rightful or virtuous choices, but also requires us to put those choices into practice by attempting to realize the goals or ends that they entail in the arena of action, that is, nothing less than the realm of spatial, temporal, and causal nature in which we live and act."

"Indeed, once Kant had discovered the Humean problem of skepticism about the universality and necessity of first principles, he generalized it not only to the first principles of “speculative philosophy,” that is, theoretical cognition, but also to the first principle of practical philosophy, the fundamental principle of morality. Thus, from a methodological point of view, Kant’s project in philosophy became that of undermining both Humean and Pyrrhonian skepticism in both theoretical and practical philosophy, and, much more incidentally, along the way refuting Cartesian skepticism about external objects as a nagging but by no means central problem in theoretical philosophy."