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Rh "new principle," that "great discovery," that theory of the non-existence of matter, which was to be one of the three important fixed ideas of his life (the other two, as we shall see, were his scheme for the evangelization of the American Indians and his belief in the virtues of tar-water). The Essay towards a New Theory of Vision (1709), in which the new principle is applied somewhat timidly to the sensations of sight, belongs to these years. Soon after this came the Treatise concerning the Principles of Human Knowledge (1710), in which the inconceivability of a material substance is demonstrated and defended at great length, and the Three Dialogues between Hylas and Philonous (1713), which are the polite manifesto of immaterialism. The great principle, presented as the best philosophic preventive against the plagues of skepticism and immorality, is thus brought within the range of parlor vision.

In 1713, with Berkeley's journey to London, begins the period of his mundane and wandering life. The young Irishman makes acquaintances, becomes the friend of Swift, who presents him at court, continues in Steele's Guardian his campaign against free-thinkers, and all at once sets out for Sicily in the suite of Lord Peterborough. In 1714 he was again in London, but he soon left to accompany the son of Bishop Ashe to France and Italy. This second journey lasted