In the metaphysical elements of aesthetics the various nonmoral feelings are to be made use of; in the elements of moral metaphysics the various moral feelings of men, according to the differences in sex, age, education, and government, of races and climates, are to be employed.
In man (as the only rational creature on earth) those natural capacities which are directed to the use of his reason are to be fully developed only in the race, not in the individual.
If an offender has committed murder, he must die. In this case, no possible substitute can satisfy justice. For there is no parallel between death and even the most miserable life, so that there is no equality of crime and retribution unless the perpetrator is judicially put to death.
We assume a common sense as the necessary condition of the universal communicability of our knowledge, which is presupposed in every logic and every principle of knowledge that is not one of skepticism.
Enlightenment is man's leaving his self-caused immaturity. Immaturity is the incapacity to use one's intelligence without the guidance of another. Such immaturity is self-caused if it is not caused by lack of intelligence, but by lack of determination and courage to use one's intelligence without being guided by another. Sapere Aude! Have the courage to use your own intelligence! is therefore the motto of the enlightenment.
A philosophical attempt to work out a universal history according to a natural plan directed to achieving the civic union of the human race must be regarded as possible and, indeed, as contributing to this end of Nature.
The infinitude of creation is great enough to make a world, or a Milky Way of worlds, look in comparison with it what a flower or an insect does in comparison with the Earth.
Even the song of birds, which we can bring under no musical rule, seems to have more freedom, and therefore more for taste, than a song of a human being which is produced in accordance with all the rules of music; for we very much sooner weary of the latter, if it is repeated often and at length. Here, however, we probably confuse our participation in the mirth of a little creature that we love, with the beauty of its song; for if this were exactly imitated by man (as sometimes the notes of the nightingale are) it would seem to our ear quite devoid of taste.
All the interests of my reason, speculative as well as practical, combine in the three following questions: 1. What can I know? 2. What ought I to do? 3. What may I hope?