It is easy to obtain confirmations, or verifications, for nearly every theory-if we look for confirmations. Confirmations should count only if they are the result of risky predictions... A theory which is not refutable by any conceivable event is non-scientific. Irrefutability is not a virtue of a theory (as people often think) but a vice. Every genuine test of a theory is an attempt to falsify it, or refute it.

It is impossible to speak in such a way that you cannot be misunderstood.

Optimism is a duty. The future is open. It is not predetermined. No one can predict it, except by chance. We all contribute to determining it by what we do. We are all equally responsible for its success.

It is not his possession of knowledge, of irrefutable truth, that makes the man of science, but his persistent and recklessly critical quest for truth.

Contrary to the outstanding work of art, outstanding theory is susceptible to improvements.

The quest for precision is analogous to the quest for certainty and both - precision and certainty are impossible to attain.

Our belief in any particular natural law cannot have a safer basis than our unsuccessful critical attempts to refute it.