A tautology's truth is certain, a proposition's possible, a contradiction's impossible.

Is it even always an advantage to replace an indistinct picture by a sharp one? Isn't the indistinct one often what we need?

Black seems to make a colour cloudy, but darkness doesn't. A ruby could thus keep getting darker without ever becoming cloudy; but if it became blackish red, it would become cloudy.

Since social relationships are always ambiguous, since my thought is only a unit, since my thoughts create rifts as much as they unite, since my words establish contacts by being spoken and create isolation by remaining unspoken, since an immense moat separates the subjective certitude that I have for myself from the objective reality that I represent to others, since I never stop finding myself guilty even though I feel I am innocent.

Believers who have formulated such proofs [for God's existence] ... would never have come to believe as a result of such proofs

What can be shown, cannot be said.

It is possible--indeed possible even according to the old conception of logic--to give in advance a description of all 'true' logical propositions. Hence there can never be surprises in logic.