All our thoughts and concepts are called up by sense-experiences and have a meaning only in reference to these sense-experiences. On the other hand, however, they are products of the spontaneous activity of our minds; they are thus in no wise logical consequences of the contents of these sense-experiences. If, therefore, we wish to grasp the essence of a complex of abstract notions we must for the one part investigate the mutual relationships between the concepts and the assertions made about them; for the other, we must investigate how they are related to the experiences.

When forced to summarize the general theory of relativity in one sentence: Time and space and gravitation have no separate existence from matter.

Women always worry about things that men forget; men always worry about things women remember.

It is an outcome of faith that nature-as she is perceptible to our five senses-takes the character of such a well formulated puzzle.

The most beautiful thing we can experience is the mysterious. It is the source of all true art and science.

We are boxed in by the boundary conditions of our thinking

Between the difficulties, one hides the opportunity