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.
A man was meant to be doubtful about himself, but undoubting about the truth; this has been reversed. Nowadays the part of a man that a man does assert is exactly the part he ought not to assert - himself. The part he doubts is exactly the part he ought not to doubt - the Divine Reason.
you must be careful never to allow doubt to paralyze you. always take the decisions you need to take, even if you're not sure you're doing the right thing. You'll never go wrong if, when you make a decision, you keep in mind an old German proverb: 'The devil is in the detail.' Remember that proverb and you'll always be able to turn a wrong decision into a right one.
The probability that we face global warming caused by fossil fuels is now so overwhelming that it is legitimate to doubt the motives of those who deny it
When in doubt, tell the truth. That maxim I did invent, but never expected it to be applied to me. I did say, "When you are in doubt," but when I am in doubt myself I use more sagacity.