Philosophy Certainty
Ali Hasan
  • LAST MODIFIED: 24 May 2017
  • DOI: 10.1093/obo/9780195396577-0344


No single concept or kind of state underlies all the ordinary uses of the term certain and its cognates. First, we can distinguish between absolute and relative certainty. When we ask whether someone is “certain” that p, we seem to use the term as an absolute, admitting of no degrees, like flat—nothing is only somewhat flat. But we often use the term in a relative sense, as when we say that something is “very certain” or that one thing is “more certain” than another. We can also distinguish between psychological and epistemic certainty. To be psychologically certain is, roughly, to be sure, confident, or without doubt about something—as when one says, “I was certain he was innocent, but it was all wishful thinking.” Other times, we intend to refer to an epistemic property or state and not (or not merely) a psychological one. When I say I am certain that triangles have exactly three sides, I am not (or not merely) saying that I am confident but that my confidence is appropriate: I am in an ideal epistemic position with respect to it. Various accounts or conceptions of epistemic certainty have been offered. Some threaten to collapse the distinction between psychological and epistemic certainty (e.g., the conception of epistemic certainty as indubitability), but there are competing epistemic conceptions that are clearly distinct from a purely psychological conception (e.g., the conception of epistemic certainty as infallibility or incorrigibility). Certainty is interesting in part due to its potential connections to knowledge and skepticism. Some arguments seem to show that knowledge requires absolute certainty. But there are very few propositions we can be certain about and so very little that we know. We might attempt to avoid this skeptical result either by denying that knowledge requires certainty or holding that we do enjoy the sort of certainty required. Some arguments might seem to show that all knowledge must rest on a foundation of certainties, even if knowledge in general does not require certainty. But some may worry that even when it comes to foundational beliefs, few are certain, and this too would seem to imply that we know very little. We might attempt to avoid the skeptical result by claiming either that knowledge does not require a foundation of certainties or that we do in fact enjoy certainty when it comes to the foundations of knowledge.

Psychological, Epistemic, and Moral Certainty

Contemporary philosophers typically distinguish between psychological and epistemic certainty. For a belief to be psychologically certain for some subject is, roughly, for the subject to have complete or unshakable conviction in its truth. For a belief to be epistemically certain for some subject is, roughly, for it to have the highest epistemic status possible. Philosophers have focused more on epistemic certainty than psychological certainty. It seems that psychological certainty is neither necessary nor sufficient for epistemic certainty. Reed 2011 argues that a belief may be psychologically certain without having much, if anything, going for it evidentially or epistemically, and, conversely, that a belief may be epistemically certain and yet the subject may have doubts, perhaps irrational ones, that lower her confidence or shake her conviction in its truth. Philosophers sometimes also speak of “moral” or “practical” certainty. Descartes 1985 takes a belief to be morally certain has when it is rational or justified enough to be relied upon in moral or practical deliberation, or in deciding what to do. Similarly, Locke 2015 takes a proposition to be practically certain if it is close enough to absolute epistemic certainty for practical purposes. He argues that assuming that p is (rather than engage in probabilistic reasoning regarding p) is rationally permissible when it is practically certain. So understood, moral or practical certainty is epistemic in nature, though it seems to fall short of epistemic certainty. We can also distinguish between certainty at a time and lasting certainty. While philosophers often focus on the conditions required for a belief to be certain at a time, Descartes recognized that it might be certain to one that p at one time (while one has a clear and distinct perception that p) but not another (when one no longer has or is not attending to this clear and distinct perception). Descartes searched for a kind of lasting and stable certainty that was psychological and epistemic. Reed 2011 points out, however, that we do not seem capable of the kind of lasting immunity from doubt that this ideal of certainty requires.

  • Descartes, Rene. The Philosophical Writings of Descartes. Vol. 2. Edited by J. Cottingham, R. Stootfhoff, and D. Murdoch. Cambridge, UK: Cambridge University Press, 1984.

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    In his Meditations (first published in 1641), Descartes distinguishes between certainty that p grounded in a clear and distinct perception that p (cognitio), and the lasting certainty that p that one can have even when one is no longer attending to the clear and distinct perception that p (scientia). See “Meditations on First Philosophy” (pp. 1–62) and “Objections and Replies” (pp. 63–398). In particular, see “Third Mediation” (pp. 24–36), “Fifth Meditation” (pp. 44–49), and “Author Replies to the Second Set of Objections” (pp. 93–120). See also Historical Discussions.

  • Descartes, Rene. “The Principles of Philosophy, Part IV.” In The Philosophical Writings of Descartes. Vol. 1. Edited by J. Cottingham, R. Stoothoff, and D. Murdoch, 267–292. Cambridge, UK: Cambridge University Press, 1985.

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    Includes a brief discussion of moral certainty versus absolute certainty (pp. 289–90 and n. 2). Descartes characterizes moral certainty as certainty sufficient for ordinary practical purposes, or for application to ordinary life.

  • Klein, Peter D. “Absolute Certainty in This World.” In Certainty: A Refutation of Scepticism. By Peter D. Klein, 115–210. Minneapolis: University of Minnesota Press, 1981.

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    Discusses the distinction between psychological and epistemic certainty and offers a useful list of desiderata for any adequate account of epistemic certainty. See also Requiring Certainty without Skepticism and Indefeasibility.

  • Locke, Dustin. “Practical Certainty.” Philosophy and Phenomenological Research 90.1 (2015): 72–95.

    DOI: 10.1111/phpr.12036E-mail Citation »

    Argues that it is rationally permissible to assume that p, rather than engage in careful probabilistic reasoning, when it is “practically certain” that p, that is, close enough to absolute epistemic certainty for practical purposes.

  • Reed, Baron. “Certainty.” In The Stanford Encyclopedia of Philosophy. Edited by Edward N. Zalta. Winter 2011.

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    An excellent encyclopedia entry on certainty, first published in 2008. It provides an accessible introduction to the topic, discusses different kinds of certainty (psychological, moral, and epistemic), competing conceptions of epistemic certainty, and the distinction between certainty at a time and certainty over time. Also see Conceptions of Epistemic Certainty; Indubitability; Infallibility; Highest Degree of Justification; and Indefeasibility.

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