"Certainly there are things worth believing. I believe in the brotherhood of man and in personal originality. But if you asked me to prove what I believe, I couldn't. You can spend your whole life trying to prove what you believe; you may hunt for reasons, but it will all be in vain. Yet our beliefs are like our existence; they are facts. If you don't yet know what to believe in, then try to learn what you feel and desire."

“Everything should be made as simple as possible, but not simpler.”

“The most simple picture one can form about the creation of an empirical science is along the lines of an inductive method. Individual facts are selected and grouped together such that their lawful connection becomes clearly apparent. ... The truly great advances in our understanding of nature originated in a manner almost diametrically opposed to induction. The intuitive grasp of the essentials or a large complex of facts leads the scientist to the postulation of a hypothetical basic law, or several such basic laws. From the basic laws (system of axioms) he derives his conclusions as completely as possible in a purely logically deductive manner. These conclusions, derived from the basic laws (and often only after time-consuming developments and calculations), can then be compared to experience, and in this manner provide criteria for the justification of the assumed basic law.”

“Noch eine Art Anwendung des Relativitätsprinzips zum Ergötzen des Lesers: Heute werde ich in Deutschland als "deutscher Gelehrter", in England als "Schweizer Jude" bezeichnet; sollte ich aber einst in die Lage kommen, als "bète noire" präsentiert zu werden, dann wäre ich umgekehrt für die Deutschen ein „Schweizer Jude", für die Engländer ein "deutscher Gelehrter".”

“Liebe Mutter! Heute eine freudige Nachricht. H. A. Lorentz hat mir telegraphiert, dass die englischen Expeditionen die Lichtablenkung an der Sonne wirklich bewiesen haben.”

“"The physical world is real." That is supposed to be the fundamental hypothesis. What does "hypothesis" mean here? For me, a hypothesis is a statement, whose truth must be assumed for the moment, but whose meaning must be raised above all ambiguity. The above statement appears to me, however, to be, in itself, meaningless, as if one said: "The physical world is cock-a-doodle-do." It appears to me that the "real" is an intrinsically empty, meaningless category (pigeon hole), whose monstrous importance lies only in the fact that I can do certain things in it and not certain others.”

“Geh Recht viel spazieren, dass Du Recht gesund wirst und lies nicht gar zu viel sondern spar Dir noch was auf bis Du gross bist.”