Although it is difficult to exaggerate the similarities between the philosophical doctrines of contemporary scientific empiricists and those which were expounded by Sextus Empiricus, the Greek physician and sceptic of the third century A. D., Sextus seems to have been neglected by most historians of empiricism. An account of his position may be of some pertinence at the present time, for a striking parallel can be drawn without any distortion. His most significant contributions are: first, the positivistic and behavioristic theory of signs which he opposed to the metaphysical theory of the Stoics; secondly, his discussion of phenomenalism and its relation to common sense claims to knowledge; and, thirdly, his account of the controversy over the principle of extensionality in logic, where the anticipation of contemporary doctrines is perhaps most remarkable.