Once in one's life, everything should be called into doubt.
Illius omnia in dubium revocare semel in vita.

Philosophers
René Descartes
French philosopher-mathematician born in 1596 who launched modern thought with "I think, therefore I am." His methodical doubt rebuilt knowledge on certainty, while his coordinate system fused geometry and algebra.
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René Descartes's Other Quotes
Related Quotes
The seeker after truth is not one who studies the writings of the ancients and, following his natural disposition, puts his trust in them, but rather the one who suspects his faith in them and questions what he gathers from them.
-- Ibn al-Haytham
The duty of the man who investigates the writings of scientists, if learning the truth is his goal, is to make himself an enemy of all that he reads.
-- Ibn al-Haytham
Fables should be taught as fables, myths as myths, and miracles as poetic fancies.
-- Hypatia
To teach superstitions as truth is a most terrible thing.
-- Hypatia
A great many people think they are thinking when they are merely rearranging their prejudices.
-- William James
I would rather have questions that can't be answered than answers that can't be questioned.
-- Richard Feynman