Scepticism concerns doubt, primarily doubt about what is true. So construed, the sceptic is not proposing that truth is just subjective opinion in the way that the relativist is. Indeed, what is motivating scepticism is rather the worry that our beliefs might not be true in an objective sense. However, scepticism can slide into relativism if the former becomes extensive enough. ‘What is scepticism?’ asks how we would differentiate between a healthy scepticism that targets only specific claims, and a more generalized radical scepticism that has pernicious consequences. It considers different types of knowledge—propositional, ability, and perceptual—and why it might matter to us that we do have the widespread knowledge that we take ourselves to have.