The only rigorous method, the only one that enables us to test an opinion against reality, is based on the clear recognition that opinions come first[as opposed to facts]—and that this is the way it should be. Then no one can fail to see that we start out with untested hypotheses—in decision-making as in science the only starting point. We know what to do with hypotheses—one does not argue them; one tests them. One finds out which hypotheses are tenable, and therefore worthy of serious consideration, and which are eliminated by the first test against observable experience.

-- Peter Drucker The Effective Executive