Much of the critical discussion around certainty in Descartes’ work centers on the relationship between “the quest for certainty” and “the search for truth”; this issue is intertwined with our understanding of clarity and distinctness, the method of doubt, and the alleged Cartesian Circle (see Circle, Cartesian). An important prior question concerns Descartes’ own conception of certainty; this is the main focus of the present entry.

Certainty for Descartes comes in degrees or levels; Descartes’ technical terminology for these degrees was to some extent shared by his contemporaries, in particular the terms “moral certainty” and “metaphysical certainty.” There are debates about how to understand both of these terms.

Moral certainty is sometimes treated in the literature simply as probability; the correctness of this identification depends in part on what conception of probability one has in mind, but note that Arnauld and Nicole (1996, 264, 270) explicitly distinguish between moral certainty and probability. (Probability is what the man of good sense must fall back on if moral certainty is unavailable.) It seems less potentially misleading to suggest that a morally certain proposition is one that is “beyond all reasonable doubt.” (The courtroom flavor of this phrase is intentional: Scholastic paradigms of moral certainty tend to focus on testimony, and Descartes himself strikingly uses the image of the mental faculties as witnesses; see Baker 2000. Even now, American juries are often instructed to seek verdicts “beyond a reasonable doubt and to a moral certainty.”) Moral certainty is “sufficient to regulate our behaviour, or … measures up to the certainty we have on matters relating to the conduct of life which we never normally doubt” (AT VIIIB 327, CSM I 289 n.2). Descartes’ example of “Rome is a city in Italy” (AT VIIIA 327, CSM I 290) is a Scholastic staple.