certainty and doubt Quotes

I have throughout been curious about how much we can be said to know and with what degree of certainty or doubtfulness.

Everyone who observes himself doubting observes a truth, and about that which he observes he is certain; therefore he is certain about a truth. Everyone therefore who doubts whether truth exists has in himself a truth on which not to doubt.... Hence one who can doubt at all ought not to doubt the existence of truth.

Is there any knowledge in the world which is so certain that no reasonable man could doubt it?

They [scientists of centuries past] call on God only from the lonely and precarious edge of incomprehension. Where they feel certain about their explanations, however, God gets hardly a mention.

Every increase in your knowledge is a simultaneous decrease. You learn and you unlearn at the same time. A new certainty is a new doubt as well.

Pain is truth; all else is subject to doubt.

If you tried to doubt everything you would not get as far as doubting anything. The game of doubting itself presupposes certainty.

I've gone through most of my life not believing anything. Either I know or I don't know, or I think.

For where is the man that has incontestable evidence of the truth of all that he holds, or of the falsehood of all he condemns; or can say that he has examined to the bottom all his own, or other men's opinions? The necessity of believing without knowledge, nay often upon very slight grounds, in this fleeting state of action and blindness we are in, should make us more busy and careful to inform ourselves than constrain others.