Always regard every man as an end in himself, and never use him merely as a means to your ends [i.e., respect that each person has a life and purpose that is their own; do not treat people as objects to be exploited].
The evil effect of science upon men is principally this, that by far the greatest number of those who wish to display a knowledge of it accomplish no improvement at all of the understanding, but only a perversity of it, not to mention that it serves most of them as a tool of vanity.
After death the soul possesses self-consciousness, otherwise, it would be the subject of spiritual death, which has already been disproved. With this self-consciousness necessarily remains personality and the consciousness of personal identity.
Things which we see are not by themselves what we see ... It remains completely unknown to us what the objects may be by themselves and apart from the receptivity of our senses. We know nothing but our manner of perceiving them.
Upon the solution of this problem, or upon sufficient proof of the impossibility of synthetical knowledge a priori, depends the existence or downfall of metaphysics.
Reason must approach nature with the view, indeed, of receiving information from it, not, however, in the character of a pupil, who listens to all that his master chooses to tell him, but in that of a judge, who compels the witnesses to reply to those questions which he himself thinks fit to propose. To this single idea must the revolution be ascribed, by which, after groping in the dark for so many centuries, natural science was at length conducted into the path of certain progress.
All thought must, directly or indirectly, by way of certain characters, relate ultimately to intuitions, and therefore, with us, to sensibility, because in no other way can an object be given to us.
Manners or etiquette ('accessibility, affability, politeness, refinement, propriety, courtesy, and ingratiating and captivating behavior') call for no large measure of moral determination and cannot, therefore, be reckoned as virtues. Even though manners are no virtues, they are a means of developing virtue.... The more we refine the crude elements in our nature, the more we improve our humanity and the more capable it grows of feeling the driving force of virtuous principles.
The greatest problem for the human species, the solution of which nature compels him to seek, is that of attaining a civil society which can administer justice universally.
Often war is waged only in order to show valor; thus an inner dignity is ascribed to war itself, and even some philosophers have praised it as an ennoblement of humanity, forgetting the pronouncement of the Greek who said, 'War is an evil in as much as it produces more wicked men than it takes away.'