The classical philosophical definition of knowledge, tracing back to ancient Greek philosophy and still the standard starting point for epistemology, holds that knowing a given claim requires three distinct conditions to all be satisfied at once: the person must believe the claim, the claim must actually be true, and the person must have adequate justification for believing it. All three conditions were traditionally treated as individually necessary and, together, jointly sufficient for knowledge — meaning a claim satisfying all three automatically counted as known, under this classical account.
Each of the three conditions rules out something knowledge clearly isn't
Each condition in the classical definition does real, distinct work. Belief is required because a person can't be said to know something they don't even believe, even if that thing happens to be true; simply guessing correctly, without genuinely believing the answer, doesn't count as knowing it. Truth is required because a person can't know something false; they can sincerely believe a false claim, even with strong justification, but philosophers have generally agreed that calling a false belief "knowledge" empties the word of its usual meaning. Justification is required because a lucky true guess, believed without any real supporting evidence or reasoning, doesn't intuitively count as knowledge either, even though it satisfies both the belief and truth conditions on its own.
Together, the three conditions were meant to capture exactly what knowing requires
The classical account's central claim was that these three conditions, belief, truth and adequate justification, when all satisfied together, were not just individually necessary but also jointly sufficient: satisfying all three was supposed to be enough, on its own, to guarantee that a person genuinely knew the claim in question, with no further condition needed. This tripartite definition dominated epistemology for centuries as the standard, largely uncontroversial account of what knowledge fundamentally consists of, treated as capturing the concept's essential structure rather than as merely one competing theory among many.
What we're still unsure about
That the classical tripartite definition served as epistemology's dominant working account of knowledge for centuries, and the reasoning behind each of its three individual conditions, are well documented in the history of philosophy. What's now a genuinely unresolved, actively debated question is whether the classical definition's claim of joint sufficiency actually holds, since philosophers have constructed cases where all three classical conditions are satisfied and yet the result still doesn't intuitively feel like genuine knowledge — a problem that has kept epistemologists searching for either an additional missing condition or a substantially revised account, without full agreement yet on which replacement, if any, is correct.
This sits inside The Definition & Nature of Knowledge, one of seven topics in Epistemology, one of five domains in Philosophy, one of seventeen subjects the app can quiz you on.