He who says he hates every kind of flattery, and says it in earnest, certainly does not yet know every kind of flattery.

When an acquaintance goes by I often step back from my window, not so much to spare him the effort of acknowledging me as to spare myself the embarrassment of seeing that he has not done so.

Doubt must be no more than vigilance, otherwise it can become dangerous.

With a pen in my hand I have successfully stormed bulwarks from which others armed with sword and excommunication have been repulsed.

The natural scientists of the previous age knew less than we do and believed they were very close to the goal: we have taken very great steps in its direction and now discover we are still very far away from it. With the most rational philosophers an increase in their knowledge is always attended by an increased conviction of their ignorance.

The more experiences and experiments accumulate in the exploration of nature, the more precarious the theories become. But it is not always good to discard them immediately on this account. For every hypothesis which once was sound was useful for thinking of previous phenomena in the proper interrelations and for keeping them in context. We ought to set down contradictory experiences separately, until enough have accumulated to make building a new structure worthwhile.

First there is a time when we believe everything, then for a little while we believe with discrimination, then we believe nothing whatever, and then we believe everything again - and, moreover, give reasons why we believe.