“In science, an observer states his results along with the “probable error”; but who ever heard of a theologian or a politician stating the probable error in his dogmas, or even admitting that any error is conceivable? That is because in science, where we approach nearest to real knowledge, a man can safely rely on the strength of his case, whereas, where nothing is known, blatant assertion and hypnotism are the usual ways of causing others to share our beliefs. If the fundamentalist thought they had a good case against evolution, they would not make the teaching of it illegal.”

“What we firmly believe, if it is true, is called knowledge, provided it is either intuitive or inferred (logically or psychologically) from intuitive knowledge from which it follows logically. What we firmly believe, if it is not true, is called error. What we firmly believe, if it is neither knowledge nor error, and also what we believe hesitatingly, because it is, or is derived from, something which has not the highest degree of self-evidence, may be called probable opinion. Thus the greater part of what would commonly pass as knowledge is more or less probable opinion.”

“None of our beliefs are quite true; all have at least a penumbra of vagueness and error.”

“It is not what the man of science believes that distinguishes him, but how and why he believes it. His beliefs are tentative, not dogmatic; they are based on evidence, not on authority or intuition.”

“I have been accused of a habit of changing my opinions. I am not myself in any degree ashamed of having changed my opinions. What physicist who was already active in 1900 would dream of boasting that his opinions had not changed during the last half century? In science men change their opinions when new knowledge becomes available; but philosophy in the minds of many is assimilated rather to theology than to science. The kind of philosophy that I value and have endeavoured to pursue is scientific, in the sense that there is some definite knowledge to be obtained and that new discoveries can make the admission of former error inevitable to any candid mind. For what I have said, whether early or late, I do not claim the kind of truth which theologians claim for their creeds. I claim only, at best, that the opinion expressed was a sensible one to hold at the time when it was expressed. I should be much surprised if subsequent research did not show that it needed to be modified. I hope, therefore, that whoever uses this dictionary will not suppose the remarks which it quotes to be intended as pontifical pronouncements, but only as the best I could do at the time towards the promotion of clear and accurate thinking. Clarity, above all, has been my aim.”

“Science does not aim at establishing immutable truths and eternal dogmas; its aim is to approach the truth by successive approximations, without claiming that at any stage final and complete accuracy has been achieved.”

“One of the commonest things to do with savings is to lend them to some Government. In view of the fact that the bulk of the public expenditure of most civilized Governments consists in payment for past wars or preparation for future wars, the man who lends his money to a Government is in the same position as the bad men in Shakespeare who hire murderers. The net result of the man's economical habits is to increase the armed forces of the State to which he lends his savings. Obviously it would be better if he spent the money, even if he spent it in drink or gambling.”