Certainty is an emotion
Just that, an emotion. Often sudden, arbitrary, and against our (as opposed to our gene’s) best interest. Not a revelation nor the distillation of reason nor its conclusion — whence this fancy that reason leads somewhere? “Gut feeling” is, you guessed it, nothing but a feeling. Just as we have unique emotions about concrete things — say, lust — , we have unique emotions about abstract ideas and statements — say, certainty. Emotions, concrete or abstract, are enzymes, catalysts: they shortcircuit dillydallying, they trigger action. Ruminating all day without acting makes as little evolutionary sense as ogling all day without fucking. Hence lust, hence certainty.
That, in a nutshell, is On Being Certain’s premise, and though I have but skimmed it in one of my epic B & N skimming marathons, I was certain of its truth the moment I read it.