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.

A writer who wishes to be read by posterity must not be averse to putting hints which might give rise to whole books, or ideas for learned discussions, in some corner of a chapter so that one should think he can afford to throw them away by the thousand.

How might letters be most efficiently copied so that the blind might read them with their fingers?

If we make a couple of discoveries here and there we need not believe things will go on like this for ever. Just as we hit water when we dig in the earth, so we discover the incomprehensible sooner or later.

Erudition can produce foliage without bearing fruit.

Above all things expand the frontiers of science: without this the rest counts for nothing.

Doubt everything at least once, even the sentence "Two times two is four."