We must be skeptical even of our skepticism.

Every advance in civilization has been denounced as unnatural while it was recent

Do not fear to be eccentric in opinion, for every opinion now accepted was once eccentric.

Even when the experts all agree, they may well be mistaken.

The dictum that human nature cannot be changed is one of those tiresome platitudes that conceal from the ignorant the depths of their own ignorance.

Fools and fanatics are always so certain of themselves.

Certain characteristics of the subject are clear. To begin with, we do not in this subject deal with particular things or particular properties: we deal formally with what can be said about any thing or any property. We are prepared to say that one and one are two, but not that Socrates and Plato are two.