Start With Opinions, Not Facts
To get the facts is impossible. There are no facts unless one has a criterion of relevance. Events by themselves are not facts.
People inevitably start out with an opinion; to ask them to search for the facts first is even undesirable. They will simply do what everyone is far too prone to do anyhow: look for the facts that fit the conclusion they have already reached. And no one has ever failed to find the facts he is looking for. ...
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 -- and that is 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 as 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.
From "The Essential Drucker" (2001) page 252.