“The difference between "can" and "must" is the key to understanding the profound effects of self-interest on reasoning . . . The social psychologist Tom Gilovich studies the cognitive mechanisms of strange beliefs. His simple formulation is that when we WANT to believe something, we ask ourselves, "Can I believe it?" Then, we search for supporting evidence, and if we find even a single piece of pseudo-evidence, we can stop thinking. We now have permission to believe. We have a justification, in case anyone asks. In contrast, when we DON'T want to believe something, we ask ourselves, "Must I believe it?" Then we search for contrary evidence, and if we find a single reason to doubt the claim, we can dismiss it. You only need one key to unlock the handcuffs of "must." Psychologists now have file cabinets full of findings on "motivated reasoning," showing the many tricks people use to reach the conclusions they want to reach.”