To write brashly about some things, it is almost necessary not to know much about them.

Why does a suppurating lung give so little warning and a sore on the finger so much?

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.

Just as there are polysyllabic words that say very little, so there are also monosyllabic words of infinite meaning.

I have often noticed that when people come to understand a mathematical proposition in some other way than that of the ordinary demonstration, they promptly say, "Oh, I see. That's how it must be." This is a sign that they explain it to themselves from within their own system.

One must judge men not by their opinions, but by what their opinions have made of them.

Be attentive, feel nothing in vain, measure and compare: this is the whole law of philosophy.