Certainty vs. Certitude. Is There a Difference?

What’s the difference between certainty vs. certitude? I suspected that they’re interchangeable, but it turns out that they have a subtle but substantial difference in definition—of that, I am certain.

Certainty and certitude, share a root word: the Latin term certus, meaning “fixed” or “settled.” Certainty originally meant “pledge” or “surety,” and then developed a sense of “something that is certain,” and certitude comes from a Latin word with the same meaning. Though distinct connotations have emerged along the way, they aren’t always regarded: A certainty can be something known based on fact, while sometimes, a certitude is something that someone is convinced of based on faith. (The synonym conviction more plainly refers to what one believes instead of what one knows.) This is a helpful distinction, but it’s unfortunately unlikely ever to emerge as one a writer would use consistently.

The antonyms vary in form: uncertainty but incertitude. (Incertain and incertainty existed in Middle English but were substituted with the un- forms.)

There are other words based on certus that include the adjectival and adverbial forms of certain, certainty, and certainly. The verb certify, which means “to confirm” or “vouch for”; the noun form is certification, something that is able to be confirmed or vouched for is certifiable. (In everyday usage, one who is, in jest or mockery, thought suitable to be judged insane is referred to as certifiable.) Certificated is an adjective derived from the outdated use of certificate as a verb. The noun certificate, originally meaning “action of certifying” now has a dominant sense of “a document that supplies certification.”