In my paper the fact the XY was not equal to YX was very disagreeable to me. I felt this was the only point of difficulty in the whole scheme...and I was not able to solve it.

What we observe is not nature itself, but nature exposed to our method of questioning.

Every word or concept, clear as it may seem to be, has only a limited range of applicability.

The world thus appears as a complicated tissue of events, in which connections of different kinds alternate or overlap or combine and thereby determine the texture of the whole.

Whether we electrons, light quanta, benzol molecules, or stones, we shall always come up against these two characteristics, the corpuscular and the undular.

The one who insists on never uttering an error must remain silent.

The solution of the difficulty is that the two mental pictures which experiment lead us to form - the one of the particles, the other of the waves - are both incomplete and have only the validity of analogies which are accurate only in limiting cases.