Definition
Logic is the study of what follows from what — the discipline that asks, given some claims, what other claims they oblige us to accept. Its central object is the inference: a movement from premises to a conclusion. Logic does not tell us which premises are true; it tells us when a conclusion is properly supported by them.
Graham Priest frames logic as the theory of valid argument. Wherever people reason — in mathematics, law, science, or ordinary conversation — they rely, often unconsciously, on patterns of inference. Logic makes those patterns explicit, names them, and tests whether they actually preserve truth.
Why it matters
How it works
Logic proceeds by abstraction. It strips an argument of its particular content and examines the bare skeleton that remains — the arrangement of connectives, quantifiers, and terms. If that skeleton guarantees that true premises yield a true conclusion, the inference is valid; any argument sharing the skeleton is valid too.
Different logical systems formalise this idea in different ways, and Priest emphasises that logic is itself a living, contested field. There is no single uncontroversial answer to what follows from what — classical, relevance, and other logics disagree — which makes logic a subject of ongoing inquiry rather than a closed rulebook.