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require as useful, just, or proper; "It takes nerve to do what she did"; "success usually requires hard work"; "This job asks a lot of patience and skill"; "This position demands a lot of personal sacrifice"; "This dinner calls for a spectacular dessert"; "This intervention does not postulate a patient's consent"
a tentative insight into the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena; "a scientific hypothesis that survives experimental testing becomes a scientific theory"; "he proposed a fresh theory of alkalis that later was accepted in chemical practices"