A posteriori means knowledge that comes from experience, observation, or evidence. In Intro to Philosophy, it is the opposite of a priori knowledge, which comes from reason alone.
A posteriori knowledge is knowledge you get after looking at the world. In Intro to Philosophy, it usually means a belief or claim that depends on experience, observation, or evidence from the senses rather than on pure reason alone.
If you know that it is raining because you looked outside, that is a simple a posteriori claim. You did not figure it out just by thinking about the concept of rain, you checked the world. Philosophers use this kind of knowledge to talk about how we justify beliefs, how we test claims, and why some statements need evidence before we accept them.
This matters because philosophy does not treat all knowledge the same way. Some claims seem true just by understanding their meaning or by reasoning carefully, while other claims need contact with reality. The statement “all bachelors are unmarried” is not the same kind of claim as “there is milk in the fridge.” The first can be known without checking the world, but the second needs a look, a measurement, or some other experience.
A posteriori knowledge is central to empiricism, the view that experience is a major source of knowledge. It also shows up in everyday reasoning, science, and ordinary decision-making. When you form a belief from an experiment, an observation, a memory of a past event, or a sensory check, you are using a posteriori support.
Philosophers often compare a posteriori with a priori to make a deeper point about epistemology: some knowledge seems built from evidence gathered outward from the world, while other knowledge seems built from thought inward. That distinction is useful because it helps you sort out what kind of justification a claim needs. It also shows why knowledge can change. New observations can strengthen, weaken, or overturn what you thought you knew, which is one reason a posteriori claims stay tied to revision and review.
A posteriori is one of the basic tools for talking about justification in Intro to Philosophy. If you are reading a text on epistemology, this term helps you identify whether a philosopher thinks knowledge starts with the senses, with reason, or with some mix of both.
It also gives you a clean way to analyze examples. A philosopher might describe a scientific finding, a personal memory, or an everyday observation, and you can ask: does this claim depend on experience? If yes, it is a posteriori. That kind of sorting is useful when a class discussion turns to what makes a belief warranted instead of just guessed.
The term also connects to bigger course debates. Empiricists lean heavily on a posteriori knowledge, while rationalists usually argue that reason alone can deliver some knowledge. Once you know the difference, you can better track where thinkers disagree, especially on questions like whether sense experience is reliable, limited, or shaped by the mind.
A posteriori knowledge also helps with philosophical skepticism. If knowledge depends on observation, then you can ask how trustworthy observation really is. That opens the door to questions about illusion, memory, error, and whether evidence can ever give certainty. In that way, the term is not just a label. It is a starting point for the whole epistemology conversation.
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view galleryA Priori
A priori is the closest contrast to a posteriori. A priori knowledge is supposed to be knowable without sensory experience, often through logic, definition, or reason alone. Philosophers use the pair to separate claims that need evidence from claims that can be justified by thought. If a professor asks you to classify a statement, this is usually the first distinction to check.
Empiricism
Empiricism says experience is a major source of knowledge, so it fits naturally with a posteriori claims. When an empiricist talks about observation, experiment, or sensory evidence, they are working in the same lane as a posteriori justification. If you are analyzing a thinker, asking whether they trust experience more than pure reason can tell you a lot about their epistemology.
Rationalism
Rationalism pushes in the other direction by emphasizing reason as a source of knowledge. That makes it a useful comparison for a posteriori knowledge, because the tension between the two is one of the classic epistemology debates. In class, you may be asked whether a philosopher thinks some truths can be known independently of experience, which is a rationalist-style move.
Propositional Knowledge
Propositional knowledge is knowledge that something is the case, like knowing that the lights are off or that a theory has a certain result. A posteriori is one way to justify propositional knowledge, since many true statements about the world are only known through observation. This connection helps when you are asked to explain not just what is known, but how it is known.
A quiz question might give you a claim and ask whether it is a priori or a posteriori. To answer well, check whether the claim depends on experience, observation, or evidence from the senses. If it does, label it a posteriori and explain the source of justification in one clear sentence.
You may also use it in short-answer or essay responses about epistemology. When a philosopher argues that knowledge starts with experience, name the claim as a posteriori and connect it to empiricism. If the prompt compares reason and evidence, a sharp answer will explain why some beliefs need to be checked against the world instead of being true by definition alone.
A posteriori and a priori are often mixed up because both are about knowledge, but they come from different sources. A posteriori knowledge depends on experience or observation, while a priori knowledge is justified without needing to check the world. If you can only verify the claim by looking, measuring, or testing, it is a posteriori.
A posteriori knowledge is knowledge based on experience, observation, or empirical evidence.
In Intro to Philosophy, the term usually shows up in epistemology, where philosophers ask how we know what we know.
The main contrast is with a priori knowledge, which is justified without sensory experience.
A posteriori claims are common in everyday life and science because they depend on checking the world.
The term connects closely to empiricism, since empiricists give experience a central role in knowledge.
A posteriori is knowledge that comes from experience or observation. In Intro to Philosophy, it is used in epistemology to describe claims that depend on evidence from the world, not just on reason or definitions. If you have to look, test, or observe to know something, that is usually a posteriori.
A posteriori knowledge comes after experience, while a priori knowledge is justified without experience. A claim like “there is a dog in the yard” needs observation, so it is a posteriori. A claim like “a triangle has three sides” is typically treated as a priori because you can know it from the concept itself.
Knowing that it rained last night because the sidewalk is wet is a classic example. You are using sensory evidence to support the claim. In philosophy class, scientific observations, personal memories, and reports about the world are all common examples of a posteriori knowledge.
Epistemology asks how knowledge is justified, and a posteriori knowledge shows one major way that happens. It lets philosophers compare experience-based knowledge with reason-based knowledge and ask which is more reliable. That comparison leads into bigger debates about empiricism, skepticism, and the limits of observation.