Can We Know Anything for Certain?

Can We Know Anything for Certain?

The question of whether we can know anything for certain is the foundational question of epistemology. It is also, arguably, the most unsettling. If the answer is no — if certainty is impossible — then the entire edifice of human knowledge rests on shifting sand. If the answer is yes — if at least some things can be known with certainty — then the follow-up question is: what are those things, and how do we know them?

The question has been asked in different forms throughout the history of philosophy. Ancient skeptics asked whether any belief is more justified than its opposite. Descartes asked whether there is anything that survives the most radical doubt. Hume asked whether our most fundamental beliefs — in causation, in the external world, in the uniformity of nature — can be rationally justified. Contemporary epistemologists ask whether knowledge requires certainty at all, or whether a more modest standard of justification is sufficient.

What makes the question difficult is that certainty seems both necessary and impossible. It seems necessary because if a belief can be doubted, how can it count as knowledge? It seems impossible because, as skeptics have shown, every belief can be doubted — perception can be illusory, memory can be false, reason can be misled, and even the existence of the external world can be questioned. Resolving this tension is one of the central tasks of epistemology.

Key Ideas

The first key idea is radical skepticism. The radical skeptic argues that certainty is impossible because any belief can be doubted. The argument typically proceeds by raising skeptical scenarios that are logically possible but empirically absurd: maybe I am dreaming, maybe I am a brain in a vat being fed false experiences, maybe an evil demon is systematically deceiving me about everything. These scenarios cannot be refuted by evidence, because any evidence I might cite is itself part of the deceived experience. The radical skeptic concludes that since any belief can be doubted, no belief is certain, and therefore no belief constitutes knowledge (if knowledge requires certainty).

The second key idea is the Cartesian response. Descartes, in his Meditations on First Philosophy (1641), used radical doubt as a methodological tool. He doubted everything that could be doubted — sensory experience (which can be illusory), mathematical beliefs (which an evil demon might make me believe falsely), even the existence of the external world. But he found one thing that could not be doubted: the existence of the thinking self. Even if I am deceived about everything, I must exist in order to be deceived. Cogito ergo sum — I think, therefore I am. This gave Descartes one certain truth from which he attempted to rebuild the edifice of knowledge. The cogito remains the most famous attempt to answer the skeptic by finding an indubitable foundation.

The third key idea is the problem of induction. David Hume argued that our belief that the future will resemble the past — the basis of all empirical reasoning — cannot be rationally justified. We observe that the sun has risen every day in the past, and we infer that it will rise tomorrow. But this inference assumes the uniformity of nature (the future will resemble the past), and that assumption cannot be justified by observation (we have not observed the future) or by reason (there is no logical contradiction in the sun not rising). Hume concluded that induction is a psychological habit, not a rationally justified inference. This means that even our best-supported empirical generalizations lack deductive certainty — they are probably true, but not certainly true.

The fourth key idea is the Moorean response. G. E. Moore, in his 1939 essay "Proof of an External World," offered a surprisingly simple response to skepticism. He held up his hand and said "Here is a hand." The argument is that the premise "here is a hand" is more certain than any skeptical premise that might be used to challenge it. If the skeptic says "you might be dreaming," Moore's response is that the evidence for the existence of his hand (he can see it, feel it, move it) is stronger than the evidence for the dreaming hypothesis. Moore's approach is not a refutation of skepticism in the deductive sense — it does not prove that the dreaming hypothesis is false — but it is a challenge to the skeptic's claim that we should abandon common-sense beliefs in the face of skeptical possibilities.

The fifth key idea is the rejection of the certainty requirement. Most contemporary epistemologists hold that knowledge does not require absolute certainty. The classical definition of knowledge as justified true belief does not mention certainty — it requires justification, which can come in degrees. A belief can be well-justified (supported by strong evidence, produced by a reliable process) without being indubitable (impossible to doubt). On this view, the skeptic's challenge is based on a false assumption — that knowledge requires the elimination of all possible doubt. If we lower the bar from certainty to justification, the skeptic's challenge loses its force, and ordinary knowledge becomes possible.

Historical Background

The question of certainty has been central to epistemology since its beginnings. In ancient Greece, the Pyrrhonian skeptics, led by Pyrrho and later systematized by Sextus Empiricus, recommended suspending judgment on all questions that go beyond immediate appearances. The goal was not to prove that knowledge is impossible but to achieve tranquility (ataraxia) by giving up the anxiety of dogmatic belief. The Academic skeptics, associated with the later Platonic Academy, took a stronger position: they argued that knowledge is impossible and that the wise person should follow probability rather than claim certainty.

Augustine, in the early Christian tradition, offered an early version of the cogito argument. In Against the Academicians (386 CE), Augustine argued that even if we can doubt everything else, we cannot doubt that we exist — for to doubt is to think, and to think is to exist. This argument prefigured Descartes by over twelve centuries, though it was less developed and less influential.

Descartes's Meditations gave the question of certainty its most influential treatment. Descartes was not a skeptic but a foundationalist who used skepticism as a tool. His method was to doubt everything that could be doubted, in order to find something that could not be doubted, and then to build knowledge on that foundation. The cogito provided the foundation; the existence of a non-deceiving God provided the bridge to knowledge of the external world; and the clear and distinct ideas of mathematics and geometry provided the model for certain knowledge. Descartes's program was enormously influential, but it faced serious challenges — particularly the "Cartesian circle" (the apparent circularity of using God's existence to guarantee the reliability of clear and distinct ideas, while using clear and distinct ideas to prove God's existence).

Hume's skepticism was different from Descartes's. Hume did not use doubt as a methodological tool but argued that the foundations of empirical reasoning are not rationally justified. Hume's skepticism about induction, causation, and the external world was not meant to destroy knowledge but to reveal its psychological rather than rational foundations. Kant was famously "awakened from his dogmatic slumber" by Hume and developed the critical philosophy to rescue the possibility of synthetic a priori knowledge.

In the twentieth century, the question of certainty was reshaped by several developments. The logical positivists sought certainty in logic and sense-data, but their program collapsed under internal criticism. Wittgenstein's later philosophy challenged the very framework of the skeptic, arguing that doubt presupposes a framework of certainty — you cannot doubt everything at once, because doubt requires some things to be settled. The externalist turn in epistemology (reliabilism, proper functionalism) shifted the focus from the knower's internal access to justification to the external reliability of the belief-forming process, undermining the skeptic's assumption that justification must be accessible.

Contemporary Relevance

The contemporary relevance of the question of certainty extends beyond academic philosophy. In everyday life, people constantly navigate between certainty and uncertainty — deciding when to act on available evidence, when to seek more information, when to accept risk. The philosophical analysis of certainty and skepticism provides a framework for thinking about these decisions.

In the domain of science, the question of certainty is directly relevant to the communication of scientific findings. Science does not produce absolute certainty — all scientific conclusions are provisional and subject to revision. But this does not mean that all scientific conclusions are equally uncertain. The misuse of scientific uncertainty — treating the provisional nature of scientific knowledge as a reason to dismiss well-supported findings — is a practical form of skepticism that has serious consequences, from climate denial to vaccine refusal.

In law, the question of certainty is built into the standards of proof. "Beyond a reasonable doubt" does not mean absolute certainty — it means a very high level of confidence. The legal system's recognition that certainty is unattainable, but that high confidence is sufficient for action, reflects a pragmatic response to the skeptical challenge.

In the domain of artificial intelligence, the question of certainty takes a new form. AI systems produce outputs with varying levels of confidence, and users must decide when to trust those outputs. The philosophical framework for thinking about certainty and uncertainty — including the distinction between subjective confidence and objective reliability — is directly applicable to the design and use of AI systems.

The broader lesson is that while absolute certainty may be unattainable, this does not lead to epistemic paralysis. We can have knowledge without certainty — reliable, well-justified beliefs that are good enough for action, even if they fall short of indubitability. The skeptic's challenge is valuable not because it shows that knowledge is impossible but because it forces us to be clearer about what we claim to know and on what basis. The question "can we know anything for certain?" may have the answer "no," but the question "can we know anything?" has the answer "yes" — and that is enough.

Sources

• Stanford Encyclopedia of Philosophy, "Epistemology."
• Stanford Encyclopedia of Philosophy, "Skepticism."
• Internet Encyclopedia of Philosophy, "René Descartes: Skepticism."
• Descartes, R. (1641). Meditations on First Philosophy. (Available in numerous scholarly editions.)
• Hume, D. (1739). A Treatise of Human Nature. (Available in numerous scholarly editions.)
• Moore, G. E. (1939). "Proof of an External World," Proceedings of the British Academy, 25, 273-300.
• Stroud, B. (1984). The Significance of Philosophical Scepticism. Oxford University Press.

Related Topics

• Epistemology — The foundational study of knowledge within which the question of certainty arises.
• Skepticism — The philosophical tradition that challenges the possibility of knowledge.
• Exploring Epistemology — A broader introduction to epistemological concepts.
• Exploring the Limits of Human Knowledge — Examines the boundaries of what we can know.
• Descartes vs. Hume — A direct comparison of two foundational approaches to certainty and knowledge.