What Is A Posteriori Knowledge?

What Is A Posteriori Knowledge?

A posteriori knowledge is knowledge that comes from experience. The Latin phrase a posteriori means "from what comes after" — after experience. When you know that it is raining because you looked out the window, that the stove is hot because you touched it, or that a medicine works because clinical trials proved it, your knowledge is a posteriori. It is justified by observation, not by pure reasoning.

This contrasts with a priori knowledge, which is justified by reason alone. You can know that "all bachelors are unmarried" just by understanding the words — no experience needed. But you cannot know that "there are bachelors in Berlin" without checking. The first is a priori; the second is a posteriori. Most of what we know about the actual world — as opposed to what we know about concepts and definitions — is a posteriori. We learn through seeing, hearing, touching, measuring, and experimenting.

The distinction matters because the two kinds of knowledge have different properties. A priori truths are typically necessary — they could not have been otherwise. It is necessarily true that 7 + 5 = 12; there is no possible world in which it is false. A posteriori truths are contingent — they happen to be true but could have been false. It is true that Paris is the capital of France, but it could have been Lyon. This means a posteriori knowledge is always fallible: since the world could have been different, our empirical beliefs could always turn out to be wrong.

Historical Background

Ancient Roots

Aristotle was the first philosopher to give experience a central role in knowledge. Against Plato, who located genuine knowledge in the grasp of eternal Forms accessible only through reason, Aristotle argued that knowledge begins with sensory perception. In De Anima and the Posterior Analytics, he described how repeated observations build up memory, then experience, and finally understanding — the grasp of universal principles that explain why things are as they are. For Aristotle, the mind starts as a blank slate, and experience writes on it.

This emphasis on experience as the starting point of knowledge became the foundation of the empiricist tradition. But Aristotle was not a pure empiricist — he held that the universal principles extracted from experience are known with a kind of necessity that goes beyond mere generalization. The tension between the empirical starting point and the aspiration to necessity would drive epistemology for centuries.

Locke and the Blank Slate

John Locke made the empiricist program explicit in An Essay Concerning Human Understanding (1690). Locke attacked the rationalist doctrine of innate ideas — the claim that the mind is born with certain ideas or principles already in place. He argued that the mind is a tabula rasa — a blank slate — and that all ideas come from two sources: sensation (experience of the external world) and reflection (experience of the mind's own operations).

On Locke's view, simple ideas like "yellow," "hot," "round," and "thinking" enter the mind through sensation and reflection. The mind then combines, compares, and abstracts from these simple ideas to form complex ones. The idea of a golden mountain, for instance, is built from the simpler ideas of "gold" and "mountain," which ultimately trace back to sensory experience. Even abstract ideas like "identity" and "cause" are derived from experience, not implanted in the mind at birth.

Locke's empiricism was revolutionary because it made experience the sole source of ideas. But it also raised a problem: if all ideas come from experience, how can we have ideas of things we have never experienced? Locke's answer — that the mind combines and abstracts from experienced ideas — was not fully convincing, and later empiricists refined and challenged his account.

Berkeley and the Challenge to Material Substance

George Berkeley took empiricism further than Locke. In A Treatise Concerning the Principles of Human Knowledge (1710), Berkeley argued that the very idea of material substance — a mind-independent physical world underlying our sensations — is incoherent. We never experience matter itself; we experience only sensations (colors, sounds, textures). The idea of a material substrate behind these sensations is not derived from experience but invented by philosophers. Berkeley concluded that the only substances that exist are minds and their ideas — a position called immaterialism or idealism.

Berkeley's argument was a reductio of empiricism taken to its logical conclusion: if all ideas come from experience, and we never experience material substance, then we have no idea of it. Most philosophers rejected Berkeley's conclusion, but his argument exposed a tension in empiricism that would become central in Hume's hands.

Hume and the Problem of Induction

David Hume delivered the most profound challenge to a posteriori knowledge. In the Enquiry Concerning Human Understanding (1748), Hume asked: what justifies our belief that the future will resemble the past? All our reasoning about unobserved matters of fact depends on this assumption. We believe the sun will rise tomorrow because it has risen before. We believe bread will nourish because it has nourished before. But what justifies the assumption that the future will resemble the past?

Hume's answer was devastating: nothing. The assumption cannot be justified by reason, because it is not a logical truth — the future could be different. And it cannot be justified by experience, because using experience to justify the assumption that the future resembles the past is circular — it presupposes the very principle it is trying to prove. This is the problem of induction: the recognition that our most fundamental empirical reasoning rests on a principle that reason cannot establish.

Hume did not conclude that we should stop believing the sun will rise. He concluded that our belief rests on habit or custom — a psychological tendency to expect continuity — rather than on rational justification. This was humbling for empiricism: it showed that the foundation of a posteriori knowledge is not as secure as the empiricists had hoped.

Key Thinkers

John Locke (1632–1704)

Locke's Essay is the foundational text of British empiricism. His central thesis — that all ideas derive from experience — established the framework within which subsequent empiricists worked. Locke distinguished between qualities of objects: primary qualities (shape, size, motion) that exist in the object itself, and secondary qualities (color, taste, smell) that exist only as powers to produce sensations in perceivers. This distinction allowed Locke to defend a realist view of the physical world while acknowledging the subjective character of sensory experience.

Locke also distinguished between nominal essences (the ideas we use to classify things) and real essences (the underlying natures of things). We know nominal essences through experience, but real essences are largely hidden from us. This means that much of our empirical knowledge is provisional and limited — we know how things appear and behave, but not what they ultimately are.

David Hume (1711–1776)

Hume was the most rigorous and skeptical of the empiricists. His Treatise of Human Nature (1739–40) and Enquiry Concerning Human Understanding (1748) pushed empiricism to its limits and exposed its deepest problems. Hume's "fork" divided all objects of reason into relations of ideas (analytic, a priori, necessary) and matters of fact (synthetic, a posteriori, contingent). This framework left no room for synthetic a priori knowledge — all substantive knowledge of the world was empirical, and all empirical knowledge was contingent and fallible.

Hume's problem of induction showed that even empirical knowledge rests on assumptions that cannot be rationally justified. His analysis of causation showed that the necessary connection we perceive between cause and effect is not observed in the objects but is a projection of the mind's habit of association. These arguments did not destroy empiricism, but they gave it a permanently skeptical edge: the empiricist must be honest about the limits of what experience can justify.

The Nature of A Posteriori Knowledge

A posteriori knowledge has several distinctive features:

It depends on experience. The justification of an a posteriori belief traces back to sensory observation, measurement, or experimentation. If you could not observe, you could not have the belief justified in this way.

It is contingent. A posteriori truths could have been false. "Water boils at 100 degrees Celsius at sea level" is true, but it could have been different if the laws of physics were different. This contingency means a posteriori knowledge is never certain in the way a priori knowledge can be.

It is fallible. Because the world could have been different, and because our senses can deceive us, a posteriori beliefs are always open to revision. A scientific theory that has been confirmed a thousand times can be overturned by new evidence. This fallibilism is not a weakness but a feature: it is what makes empirical knowledge self-correcting.

It involves induction. Most a posteriori reasoning goes beyond what has been directly observed to what has not. We observe some swans, find them white, and infer that all swans are white. This inference — from particular observations to general conclusions — is induction, and it is the engine of empirical knowledge. It is also, as Hume showed, philosophically problematic: there is no logical guarantee that the next swan will be white.

Scientific Knowledge as A Posteriori

Science is the most systematic form of a posteriori knowledge. The scientific method is designed to produce reliable empirical beliefs through controlled observation, experimentation, and peer review. But science is not simply the accumulation of observations. It involves hypotheses — claims about unobserved regularities — that are tested against experience and revised in light of evidence.

Karl Popper argued that scientific knowledge is not inductively justified at all. On Popper's view, scientists do not verify theories by accumulating confirming instances. They falsify them by searching for disconfirming evidence. A scientific theory is never proven true; it is only not yet proven false. This falsificationism was Popper's response to Hume's problem of induction: if induction cannot be justified, science should not pretend to verify theories. It should instead focus on refuting them.

Popper's view is influential but contested. Many philosophers hold that science does involve induction — not as a matter of logical proof, but as a matter of weighing evidence. A theory that has survived many severe tests is more probable than one that has not, even if it cannot be proven certain. This Bayesian approach treats scientific reasoning as a matter of updating probabilities in light of evidence, which is a form of a posteriori reasoning that takes Hume's challenge seriously without surrendering to it.

Contemporary Relevance

The question of a posteriori knowledge is not merely academic. It bears directly on how we evaluate claims in everyday life:

Evaluating testimony. Much of what we know comes from other people's reports — news, expert opinion, testimony. When is testimony a reliable source of a posteriori knowledge? The answer depends on the reliability of the source, the consistency of the reports, and the absence of distorting incentives.

Assessing scientific claims. Climate science, public health research, and economic studies all produce a posteriori claims that bear on policy. Understanding how these claims are justified — what evidence supports them, what uncertainties remain — is essential for informed citizenship.

Recognizing the limits of experience. Hume's problem of induction is not just a puzzle for philosophers. It is a reminder that empirical knowledge is always provisional. The fact that something has always happened a certain way does not guarantee it always will. This humility is a feature of good empirical reasoning, not a weakness.

Distinguishing correlation from causation. A posteriori observation can reveal that two things are correlated — they tend to occur together. But establishing that one causes the other requires more: controlled experiments, mechanistic understanding, and the elimination of alternative explanations. This distinction is at the heart of scientific reasoning and is essential for evaluating empirical claims.

How to Apply This

1. Ask what evidence supports a belief. If a belief is a posteriori, it should be traceable to observation or experimentation. If no evidence is offered, the belief may be unjustified regardless of how strongly it is held.

2. Distinguish observation from inference. What did you actually observe, and what did you infer? The gap between observation and inference is where errors creep in. Being clear about what you saw — as opposed to what you concluded — is the first step in evaluating a posteriori beliefs.

3. Take induction seriously. Most empirical reasoning involves going beyond what has been observed. This is legitimate — we cannot function without it — but it carries risk. The strength of an inductive inference depends on the quantity and quality of the observations, the diversity of the cases, and the plausibility of the underlying mechanism.

4. Be honest about fallibility. A posteriori knowledge is never certain. Holding empirical beliefs with an awareness of their fallibility is not a sign of weakness but of intellectual honesty. The scientist who says "the evidence supports this, but it could be revised" is being more truthful than one who says "this is proven."

5. Check the source. A posteriori beliefs are only as good as the observations behind them. In an era of misinformation, evaluating the reliability of sources — their track record, their incentives, their methods — is essential.

Sources

1. Internet Encyclopedia of Philosophy, "A Priori and A Posteriori." Overview of the distinction and its philosophical significance.
2. Stanford Encyclopedia of Philosophy, "John Locke." Context for the blank slate and the empiricist theory of ideas.
3. Stanford Encyclopedia of Philosophy, "David Hume." Context for the problem of induction and the analysis of causation.
4. Internet Encyclopedia of Philosophy, "Empiricism." Overview of the empiricist tradition.

Related Topics

• What is knowledge? — the broader framework in which a posteriori knowledge is situated.
• What is a priori knowledge? — the complement of a posteriori knowledge.
• What is belief? — the mental state that experience produces.
• Exploring epistemology — the broader field of which the a priori / a posteriori distinction is a part.