Scepticism in the ordinary understanding is a doubting disposition, a healthy questioning mistrustfulness of extravagant or suspect claims to knowledge. Philosophical scepticism incorporates the attitude, but is more comprehensive in its objects. A philosophical sceptic may doubt the possibility of all knowledge, as the ancient Pyrrhonists did, or question our ability to obtain specific but fundamental kinds of knowledge. Early twentieth-century philosophy, for example, was much exercised by sceptical challenges to prove the existence of the ‘external world’ and minds other than one’s own. How do I know that there are other minds when all I ever see are bodies and behaviour? How do I know that there are material objects when all I directly apprehend are subjective sense data or perceptions?