George Berkeley

Berkeley was born in Ireland and educated at Trinity College Dublin, and had written all his major philosophical works by his late twenties. The New Theory of Vision analyses how we judge distance by sight; the Principles of Human Knowledge and Three Dialogues between Hylas and Philonous set out and defend his immaterialism. Contemporaries found the ideas absurd; Dr Johnson kicked a stone and said, 'I refute it thus'.

His argument starts from Locke: we directly know only ideas. Locke said secondary qualities exist only in the mind; Berkeley said primary qualities do too, since we can never conceive an extension or shape that is not perceived. So 'material substance' is an empty word, things are collections of ideas, and their being is to be perceived. The table exists when no one is in the room because God perceives it always. He saw this not as scepticism but as a defence of common sense and religion that removed the ground from materialism and atheism.

He later threw himself into a scheme for a college in Bermuda to educate colonial youths and Native Americans, sailed for America with his new wife in 1728 and waited three years in Rhode Island for a parliamentary grant that never came. He then served eighteen years as bishop of Cloyne in Ireland, wrote Siris in praise of tar-water as medicine, and retired to Oxford, where he died. Berkeley, California, is named after him for his line 'westward the course of empire takes its way'.