Epistemology · All lessons
Epistemology is the study of knowledge, and its oldest question is deceptively simple: what does it take to truly know something, rather than merely believe it or guess correctly? This lesson works through the classic answer — knowledge as justified true belief — the famous problem that shook it, and where the sources of knowledge come from.
We believe a great many things. Some are true, some false; some we hold for good reasons, others on a whim. Knowledge seems to be a special, higher status that only some beliefs achieve. If you know that it is raining, you are not merely guessing, and you are not simply lucky to be right — there is something more secure about your relationship to that fact. Epistemology tries to say exactly what that 'something more' is.
For more than two thousand years, the standard answer — with roots in Plato — was that knowledge is justified true belief. To know that something is the case, three conditions must all be met:
Notice what is deliberately not on the list: absolute certainty. On this view you can know a great deal without being one hundred per cent certain of it, which fits how we ordinarily talk about knowledge.
The justification condition is doing important work. Suppose you guess, with no reason at all, that a tossed coin will land heads — and it does. Your belief was true, and you did believe it, but you had no good grounds, so it fails the justification test. That is exactly why we do not count a lucky correct guess as knowledge: getting the right answer is not enough if you had no business being confident.
For centuries the justified-true-belief formula looked secure. Then, in a three-page paper in 1963, the philosopher Edmund Gettier described cases that seemed to meet all three conditions and yet were clearly not knowledge.
Here is a Gettier-style example. You look across a field and see what appears to be a sheep, and form the belief, 'there is a sheep in the field.' You have good evidence, and as it happens the belief is true — there really is a sheep in the field. But what you are actually looking at is a large dog that merely looks like a sheep; the real sheep is hidden behind a rock. Your belief is true, and justified, yet it is true only by luck. Most people feel you did not really know there was a sheep there.
Cases like this suggest that justified true belief is not quite enough for knowledge — that something more is needed to rule out this kind of luck. Philosophers have proposed many repairs, and the debate is still open. The lasting lesson is how a tidy, ancient definition can be overturned by a single well-chosen counterexample — a very characteristic piece of philosophical method.
Alongside 'what is knowledge?' epistemology asks 'where do we get it?' The main proposed sources include perception (the senses), reason, memory, and testimony (what others tell us). A long-running dispute concerns how much weight to give each:
This connects to the distinction between a priori knowledge (justified independently of experience, like 'all bachelors are unmarried') and a posteriori knowledge (depending on experience, like 'water boils at 100°C at sea level').
Lurking behind all of this is the skeptic, who asks whether we know much of anything at all. How do you know you are not dreaming right now, or being deceived about the whole external world? Descartes pressed exactly this kind of doubt before trying to answer it. Skeptical challenges are valuable even if we reject them, because they force us to say clearly what our knowledge actually rests on.