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.
In the natural state no concept of God can arise, and the false one which one makes for himself is harmful. Hence the theory of natural religion can be true only where there is no science; therefore it cannot bind all men together.
Act in such a way that you treat humanity, whether in your own person or in the person of any other, never merely as a means to an end, but always at the same time as an end.
Enthusiasm is always connected with the senses, whatever be the object that excites it. The true strength of virtue is serenity of mind, combined with a deliberate and steadfast determination to execute her laws. That is the healthful condition of the moral life; on the other hand, enthusiasm, even when excited by representations of goodness, is a brilliant but feverish glow which leaves only exhaustion and languor behind.
The instruction of children should aim gradually to combine knowing and doing. Among all sciences mathematics seems to be the only one of a kind to satisfy this aim most completely.
If we could see ourselves... as we really are, we should see ourselves in a world of spiritual natures, our community which neither began at birth nor will end with the death of the body.
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.