From the Twelve Virtues of Rationality:

The fourth virtue is evenness. One who wishes to believe says, “Does the evidence permit me to believe?” One who wishes to disbelieve asks, “Does the evidence force me to believe?” Beware lest you place huge burdens of proof only on propositions you dislike, and then defend yourself by saying: “But it is good to be skeptical.” If you attend only to favorable evidence, picking and choosing from your gathered data, then the more data you gather, the less you know. If you are selective about which arguments you inspect for flaws, or how hard you inspect for flaws, then every flaw you learn how to detect makes you that much stupider. If you first write at the bottom of a sheet of paper “And therefore, the sky is green!” it does not matter what arguments you write above it afterward; the conclusion is already written, and it is already correct or already wrong. To be clever in argument is not rationality but rationalization. Intelligence, to be useful, must be used for something other than defeating itself. Listen to hypotheses as they plead their cases before you, but remember that you are not a hypothesis; you are the judge. Therefore do not seek to argue for one side or another, for if you knew your destination, you would already be there.

See also

Evenness