douter

To doubt (douter de quelque chose) — but the reflexive se douter de means almost the opposite, "to suspect/have a hunch," one of French's most well-known verb-plus-pronoun meaning flips.

• Je doute qu'il vienne ce soir. — I doubt he'll come tonight.
• Elle se doutait déjà de la réponse. — She already suspected the answer.
• Ne doutez jamais de vos capacités. — Never doubt your abilities.

Douter de (to doubt, to be unsure) and se douter de (to suspect, to have a strong hunch that something IS true) are near-opposites in confidence — mixing them up reverses the intended meaning, one of the most commonly flagged pairs in French textbooks.

• book Discours de la méthode — Descartes' 1637 treatise (and the 1641 Méditations métaphysiques) builds its whole method on systematic douter — doubting everything not absolutely certain — leading to the famous conclusion "je pense, donc je suis."