To be able to discern that what is true is true, and that what is false is false,—this is the mark and character of intelligence.

Intelligence is often mistaken for speed or vocabulary or the ability to argue any side. What it actually amounts to is a reliable sense of what is so, held even when the true thing is dull and the false thing is more attractive. Most errors are not failures of reasoning power, they are failures to admit what was already visible.

Take one belief you hold that would be inconvenient if false and go looking for the strongest case against it. Notice how much you want the answer to come out a particular way. Being able to sit with a true thing you dislike is the whole skill.