I never made one of my discoveries through the process of rational thinking
Physical concepts are free creations of the human mind, and are not, however it may seem, uniquely determined by the external world.
The heart says yes, but the mind says no.
Truth is what stands the test of experience.
The hardest thing to understand in the world is the income tax.
Knowledge exists in two forms - lifeless, stored in books, and alive, in the consciousness of men. The second form of existence is after all the essential one; the first, indispensable as it may be, occupies only an inferior position.