"You understand sleep when you are awake, not while you are sleeping. You can see mistakes in arithmetic when your mind is working properly; while you are making them you cannot see them. Good people know about both bad and evil; bad people do not know about either."

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Boswell: But, Sir is it not somewhat singular that you should happen to have Cocker's Arithmetic about you on your journey? Dr. Johnson: Why, Sir if you are to have but one book with you upon a journey, let it be a book of science. When you read through a book of entertainment, you know it, and it can do no more for you; but a book of science is inexhaustible.

— James Boswell