2. Again Berkeley seeks to show, by demolishing the distinction between primary and secondary qualities, that matter as an abstract idea has no existence. This distinction was as old as the Greek, Democritus, and was accepted by Locke. We have already described it: of a thing like a lump of sugar, the sense qualities of whiteness, roughness, sweetness, etc., are secondary because they depend upon our sensations for their existence; they are the ways in which our organisms are affected, and not true copies of things; the mathematical qualities, form, size, density, impenetrability, are primary because they exist independent of our senses and are true copies of things. Hobbes had already shown that such a distinction is erroneous, and Berkeley followed him by maintaining that all qualities are secondary. The size and impenetrability of a body depends as much on sense-perception as its sweetness and color. At some length in his _Theory of Vision_ Berkeley takes up the question of the solidity, or third dimension, of a material body, and shows that it is an inference depending on sensations arising from the convergence of the two eyes and complicated by the sensations of touch. Berkeley professed to be pleading the cause of the man in the street who wants a philosophy that is real “common sense.” He maintained that the conception of matter is only a philosophical subtlety for those philosophers who seek for something beyond perception. The man in the street wishes to explain things as he finds them, and not to seek mysterious abstractions which philosophers say in one breath that we know, and in another that we cannot know. Therefore, while Berkeley agreed with Locke’s assumption of the existence of the spiritual substance, he departed from Locke in denying the existence of a material substance. Berkeley accepted, therefore, one of the two assumptions common to the Enlightenment, but he denied the other. Now Berkeley was trying to prove a thesis. He was controlled by the ideal of his ardent religious nature to free religion from false philosophy. He felt that the foes of religion――atheism and materialism――had employed effectively abstract ideas, which had been one of the weapons of religion, against religion itself. Berkeley concentrated his attack against the traditional scholastic conception of abstract ideas in general and the abstract idea of matter in particular. Abstract ideas have no existence; the idea of a material substance is an abstract idea and therefore has no existence. Berkeley was bound from the beginning of his religious crusade to explain away the existence of material substance.