What Is Certainty in Philosophy?

What Is Certainty?

Certainty is the highest standard a belief can meet. A belief is certain when it is impossible to doubt — not merely difficult to doubt, but impossible. If a belief is certain, it cannot be false, and no rational person could withhold assent from it. Certainty stands at the top of the epistemic hierarchy: above mere belief, above justified belief, above knowledge. A person can know something without being certain of it, but a person who is certain of something also knows it.

This definition raises immediate questions. What makes a belief impossible to doubt? Is certainty a property of the belief itself, or of the believer's psychological state? Can certainty be achieved at all, or is it an unattainable ideal? These questions have driven philosophical debate from antiquity to the present.

Philosophers distinguish two kinds of certainty. Psychological certainty is a subjective feeling: the sense that a belief is beyond question. A person who is psychologically certain feels no doubt. Epistemic certainty is an objective standard: a belief that is actually beyond question, because it is necessarily true and its truth is evident. The two can come apart. A person can be psychologically certain of something false — a fanatic is certain of their ideology. And a person can be epistemically entitled to certainty without feeling it — a mathematician who has proven a theorem but has not yet checked the proof for errors.

This distinction matters because the history of philosophy is largely the history of attempts to find epistemic certainty and the skeptical responses that show how hard this is.

Historical Background

Ancient Skepticism

The quest for certainty has always been shadowed by skepticism. The ancient Greek skeptics — both Pyrrhonian and Academic — argued that certainty is unattainable. For every argument, there is a counterargument; for every appearance, there is a conflicting appearance. The Pyrrhonists concluded that the rational response is suspension of judgment — refusing to assent to any claim, including the claim that certainty is impossible.

The Pyrrhonian strategy was not to prove that knowledge is impossible but to show that for every purported piece of knowledge, there is an equally strong reason to doubt. The same tower looks round from one distance and square from another. The same water feels warm to one hand and cold to another. Since appearances conflict, we cannot trust any of them with certainty. The Pyrrhonists did not deny that we have experiences; they denied that those experiences provide certain knowledge.

Descartes and the Quest for Certainty

Rene Descartes made the quest for certainty the foundation of modern philosophy. In the Meditations on First Philosophy (1641), Descartes employed a method of radical doubt: he systematically questioned every belief he could, searching for one that was impossible to doubt. If he could find even one indubitable belief, he could use it as a foundation to rebuild the edifice of knowledge.

Descartes tested three sources of belief. First, the senses: but the senses sometimes deceive us (a distant tower looks round but is square), and if they deceive us once, we cannot trust them with certainty. Second, the reality of the external world: but perhaps I am dreaming, and the world I experience is a dream. Third, even mathematical truths: perhaps an evil demon is deceiving me about everything, including the truths of arithmetic.

The one belief that survived this radical doubt was the cogito: "I think, therefore I am." Even if an evil demon is deceiving me about everything else, I cannot doubt that I am thinking, because the very act of doubting is a thought. And if I am thinking, I must exist. This insight became the foundation of Descartes' epistemology — the one certain truth from which he attempted to reconstruct knowledge.

Descartes then argued that the existence of a non-deceiving God could be established by reason, and that God's veracity guarantees the reliability of clear and distinct ideas. This allowed him to recover trust in the senses, in mathematics, and in the external world — but only because they were grounded in the certainty of the cogito and the goodness of God. The success of this project has been debated for centuries, but the Cartesian framework — seeking certainty through the method of doubt — shaped modern epistemology.

Hume and the Limits of Certainty

David Hume challenged the Cartesian project from a different angle. In the Enquiry Concerning Human Understanding (1748), Hume argued that our most important beliefs — about causation, the external world, and the future — cannot be established with certainty. The belief that the sun will rise tomorrow, that bread will nourish, or that other people have minds is not certain; it rests on habit and custom, not rational proof.

Hume did not deny that we hold these beliefs. He argued that nature has equipped us to hold them, and that we cannot help doing so. But he insisted on intellectual honesty: we should acknowledge that these beliefs are not certain. They are reasonable in a practical sense — they have served us well — but they do not meet the standard of certainty that Descartes sought.

Hume's argument was a blow to the Cartesian project. If the foundation of empirical knowledge is habit rather than certainty, then the quest for certainty in empirical matters is futile. What remains certain, on Hume's view, are only "relations of ideas" — analytic truths like mathematics and logic, which are true by definition and carry no information about the world.

Key Thinkers

Descartes: Certainty Through Doubt

Descartes' contribution was to make certainty the goal of philosophical inquiry and to develop a method for achieving it. The method of doubt — suspending belief in everything that can be doubted — is a way of testing beliefs for certainty. Only beliefs that survive the most radical doubt qualify as certain.

The cogito was Descartes' paradigm of certainty. Its certainty does not come from the senses, from memory, or from authority. It comes from the structure of the thought itself: the act of doubting one's own existence proves that one exists, because doubting is a form of thinking, and thinking requires a thinker. This is what philosophers call self-verifying: the attempt to doubt it confirms it.

Wittgenstein: Certainty Without Foundations

Ludwig Wittgenstein transformed the debate about certainty in his posthumous work On Certainty (1969). Wittgenstein rejected the Cartesian picture in which some beliefs are certain because they are indubitable foundations. Instead, he argued that certainty functions differently: some beliefs are not held because they are justified by evidence but because they form the background against which evidence and justification operate.

Wittgenstein called these beliefs hinge propositions. They are the hinges on which our practices of doubting and justifying turn. "Here is a hand" (said while looking at one's hand) is not a belief that is justified by evidence; it is the kind of thing that must be certain for the practice of giving evidence to get started. You cannot doubt that this is a hand while trusting your eyes about anything else.

This does not mean hinge propositions are known with Cartesian certainty. Wittgenstein's point is subtler: certainty is not a property of isolated beliefs but a feature of the language games in which beliefs are embedded. To participate in a practice — science, law, everyday life — is to accept certain things as certain, not because they have been proven but because they are the framework within which proof operates.

Wittgenstein's account dissolves the Cartesian quest. There is no special class of beliefs that are certain because they are indubitable. Rather, certainty is distributed across our practices: some things are certain because doubting them would make the practice of doubting itself impossible. This is a pragmatic rather than a foundational account of certainty.

Types of Certainty

Psychological Certainty

Psychological certainty is the subjective feeling of being beyond doubt. A person who is psychologically certain has no sense of uncertainty, no lingering suspicion that they might be wrong. This feeling can be powerful and can accompany true or false beliefs.

Psychological certainty is not a reliable guide to truth. Fanatics, ideologues, and conspiracy theorists are often psychologically certain of their beliefs. A person who is certain that the Earth is flat has a psychological state of certainty, but the belief is false. This is why philosophers distinguish psychological certainty from epistemic certainty: the former is a feeling, the latter is a standard.

Epistemic Certainty

Epistemic certainty is the objective standard: a belief is epistemically certain if it cannot be false and its truth is evident to a rational person. This is the kind of certainty Descartes sought. It requires not just that the believer feels certain but that the belief is actually beyond rational doubt.

What beliefs meet this standard? Descartes thought the cogito did. Most philosophers accept that certain logical and mathematical truths are epistemically certain: "2 + 2 = 4," "nothing is both red and green all over," "if A is taller than B and B is taller than C, then A is taller than C." These truths are necessary — they could not be false — and their truth is accessible to reason.

The controversial question is whether any empirical beliefs — beliefs about the external world — can be certain. Descartes thought they could be, if grounded in the cogito and the goodness of God. Hume thought they could not. Most contemporary philosophers side with Hume: empirical beliefs are always fallible, because the senses can deceive and the world could be different from how it appears.

Moral Certainty

A third category, used by Descartes himself, is moral certainty: a belief that is so probable that it is practically certain, even if not absolutely certain. Descartes distinguished metaphysical certainty (impossible to doubt) from moral certainty (extremely probable). In practice, we often treat moral certainty as sufficient for action: we do not need metaphysical certainty that a bridge is safe before crossing it; moral certainty is enough.

This distinction is important because it allows us to act without demanding an impossible standard of certainty. The skeptic who insists on metaphysical certainty before acting would be paralyzed. Moral certainty provides a practical standard that is rigorous enough for action but acknowledges the limits of human knowledge.

Contemporary Relevance

The question of certainty remains relevant in several contexts:

Science. Scientific knowledge is not certain in the Cartesian sense. Scientific theories are always revisable in light of new evidence. This does not mean they are unreliable — the evidence for evolution, climate change, and the germ theory of disease is overwhelming — but it means they are not certain. Understanding this is important for defending science against those who exploit the fallibility of science to undermine well-established findings.

Law. Legal standards of proof distinguish different levels of certainty. "Beyond reasonable doubt" (criminal law) does not require absolute certainty; it requires that no reasonable person could doubt the conclusion. "Preponderance of the evidence" (civil law) is a lower standard. These standards acknowledge that absolute certainty is unattainable in matters of fact and that practical decisions must be made on the basis of probabilities.

Everyday reasoning. We make decisions every day based on beliefs that are not certain. We trust that the food we eat is safe, that the people we interact with are trustworthy, that the structures we enter are sound. These beliefs are not certain, but they are reasonable. Understanding the distinction between certainty and reasonable belief helps us navigate the gap between what we know and what we need to know to act.

How to Apply This

1. Distinguish certainty from confidence. Confidence is a matter of degree; certainty is all-or-nothing. Most of our beliefs are held with varying degrees of confidence, and that is appropriate. Demanding certainty where only confidence is available leads to paralysis.

2. Recognize the limits of empirical certainty. Beliefs about the external world are never certain in the Cartesian sense. This is not a reason for skepticism but for humility. Hold empirical beliefs with appropriate confidence, not false certainty.

3. Use doubt productively. The Cartesian method of doubt is a tool for testing beliefs, not an end in itself. Doubt what can be doubted, and see what survives. The beliefs that survive are the most secure, even if they are not absolutely certain.

4. Understand the role of hinge propositions. Some beliefs function as background assumptions — they are not justified by evidence but make the practice of justification possible. Recognizing these helps distinguish productive doubt from destructive skepticism.

5. Accept moral certainty for practical purposes. In most practical contexts, moral certainty — extremely high probability — is sufficient. Demanding metaphysical certainty before acting is not a sign of rigor but of a misunderstanding about how knowledge works.

Sources

1. Stanford Encyclopedia of Philosophy, "Certainty." Scholarly overview of the concept and its history.
2. Stanford Encyclopedia of Philosophy, "Descartes' Epistemology." Context for the method of doubt and the cogito.
3. Stanford Encyclopedia of Philosophy, "Wittgenstein." Context for On Certainty and the concept of hinge propositions.
4. Stanford Encyclopedia of Philosophy, "Ancient Skepticism." Context for the Pyrrhonian challenge to certainty.

Related Topics

• What is knowledge? — how certainty relates to the broader concept of knowledge.
• What is doubt? — the counterpart of certainty in the method of doubt.
• What is belief? — the mental state that certainty qualifies.
• What is justification? — the epistemic standard below certainty.