One must credit an hypothesis with all that has had to be discovered in order to demolish it.
— Jean Rostand

It is theory that decides what can be observed.
Albert Einstein assumptions quote

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Picture quote about Assumptions

There are two times in a man's life when he should not speculate: when he can't afford it, and when he can.
— Mark Twain

A speculator is a man who observes the future, and acts before it occurs.
— Bernard Baruch

Theories are always very thin and insubstantial, experience only is tangible.
— assumptions quotation by Hosea Ballou