You believe thousands of things: that the earth orbits the sun, that your friend's name is what you think it is, that the bridge you drive over will hold. Some of these beliefs you checked yourself; most you took from teachers, books or the people around you. Epistemology (from the Greek epistēmē, knowledge) is the branch of philosophy that asks how any of this counts as knowledge, and how we should form beliefs when we cannot be sure.

The central questions

• What is knowledge? What is the difference between knowing something and merely believing it, or guessing right?
• What is truth? What does it mean for a belief or statement to be true?
• What is justification? What makes a belief reasonable to hold? When is evidence good enough?
• What are the sources of knowledge? Perception, memory, reasoning, testimony from others: how far can each be trusted?
• What are the limits of knowledge? Can we know anything with certainty? The skeptic says no.

Three kinds of knowing

English uses “know” in several ways. You can know how to ride a bicycle (a skill), know a person or a city (acquaintance), and know that something is the case, such as that water boils at 100 degrees Celsius at sea level. Epistemologists focus mainly on the third kind, called propositional knowledge, because it involves claims that can be true or false. A proposition is what a statement says: the content that can be believed, doubted or denied.

Rationalism and empiricism

A long-running debate concerns where knowledge comes from. Rationalists, such as René Descartes and Gottfried Leibniz, held that reason alone can give us important knowledge, as in mathematics, where we seem to know that 7 + 5 = 12 without any experiment. Empiricists, such as John Locke and David Hume, held that our ideas and knowledge of the world come ultimately from experience. Locke compared the mind at birth to blank white paper. Knowledge we can have independently of experience is called a priori; knowledge that depends on experience is a posteriori. Most philosophers today accept some of each.

Why epistemology matters now

These may sound like abstract puzzles, but they have practical force. We are surrounded by claims: news reports, advertisements, scientific findings, social media posts, conspiracy theories. Deciding which to believe, how strongly, and whom to trust is an everyday task, and doing it badly has real costs, from health choices to elections. Democracy itself depends on citizens being able to form reasonable beliefs about shared facts.

The mathematician W.K. Clifford, in an 1877 essay called “The Ethics of Belief,” argued that “it is wrong always, everywhere, and for anyone, to believe anything upon insufficient evidence.” William James replied in “The Will to Believe” (1896) that in some cases, where a choice is unavoidable and evidence cannot settle it, we may legitimately let our hopes and commitments decide. Their debate raises a question that runs through this course: are we responsible for what we believe?

Belief comes in degrees

Ordinary talk treats belief as on or off: you believe something or you do not. But you are more confident that the sun will rise tomorrow than that your team will win on Sunday. Much modern epistemology treats belief as a matter of degree, and asks how confident we should be given our evidence. Level 6 explores this idea.

The plan of the course

We start with truth, then knowledge, then justification, then the challenge of skepticism. The second half turns practical: probability, science, and how to rely on other people wisely.

Worked example: tracing one belief

Take an ordinary belief: “The pharmacy on my street closes at 9 p.m.” Ask the epistemologist's question: how do you know?

• Perception: perhaps you once saw the hours posted on the door.
• Memory: you are relying on remembering that sign correctly, months later.
• Testimony: the sign is itself someone else's statement, and you trust it was accurate.
• Inference: you assume the hours have not changed since, because store hours usually stay the same.

Even this simple belief rests on four different sources, and each could fail. Your memory might be wrong, the sign might have been outdated, the hours might have changed. Yet it would be strange to say you do not know when the pharmacy closes. Most of what we know is like this: well supported but not guaranteed.

Now raise the stakes. You need a prescription urgently tonight, and it is 8:40. Many people find they would now say, “I think it closes at 9, but I should call to check.” The evidence has not changed, only how much depends on being right. Some philosophers argue that the standards for knowing really do rise with the stakes; others say only our willingness to act changes. Either way, the example shows how epistemology connects to everyday decisions: how sure we need to be depends partly on what we are about to do with the belief.