I feel nothing, apart from a certain difficulty in continuing to exist.

Our sun enlightens the planets that belong to him; why may not every fixed star also have planets to which they give light?

Leibniz never married; he had considered it at the age of fifty; but the person he had in mind asked for time to reflect. This gave Leibniz time to reflect, too, and so he never married.

In vain we shall penetrate more and more deeply the secrets of the structure of the human body, we shall not dupe nature; we shall die as usual.

Mathematicians are like lovers. Grant a mathematician the least principle, and he will draw from it a consequence which you must also grant him, and from this consequence another.

To despise theory is to have the excessively vain pretension to do without knowing what one does, and to speak without knowing what one says.

A true philosopher is like an elephant; he never puts the second foot down until the first one is solidly in place.