An action, to have moral worth, must be done from duty. - Immanuel Kant

For if we regard space and time as properties that must, as regards their possibility, be found in things in themselves, [...] then we really cannot blame the good Bishop Berkeley for degrading bodies to mere illusion. Nay, even our own existence, which would thus be made dependent on the self-subsistent reality of a non-entity such as time, would, along with this time, be changed into mere illusion - an absurdity of which hitherto no one has been guilty.

A plant, an animal, the regular order of nature — probably also the disposition of the whole universe — give manifest evidence that they are possible only by means of and according to ideas; that, indeed, no one creature, under the individual conditions of its existence, perfectly harmonizes with the idea of the most perfect of its kind — just as little as man with the idea of humanity, which nevertheless he bears in his soul as the archetypal standard of his actions; that, notwithstanding, these ideas are in the highest sense individually, unchangeably, and completely determined, and are the original causes of things; and that the totality of connected objects in the universe is alone fully adequate to that idea.

Chisels and hammers may suffice to work a piece of wood, but for etching we require an etcher's needle. Thus common sense and speculative understanding are both useful, but each in its own way: the former in judgments which apply immediately to experience; the latter when we judge universally from mere concepts, as in metaphysics, where sound common sense, so called in spite of the inappropriateness of the word, has no right to judge at all.