Locke proposed to find out what the mind could know by counting and tabulating the things he found in his own intelligence. This is very much like trying to understand the nature of light, by considering the blue things and green things and red things the light discloses. All bodies, it is said, which the light enables us to see, attract each other in proportion to their mass, and inversely as the squares of their distance. The law of gravity, which regulates the bodies light reveals to us, is not the law of light. We can never understand the nature of light, or the laws of light, by the study of things which light enables us to see. If all knowledge is but the sum of the impressions which the external world has made on the mind, then the cause of knowledge is matter, and knowledge is but the image or reflection of material things. Knowledge, then, would sustain the same relation to the outside world, that the shadow of a tree does to the tree. One would come as near lifting up the tree by its own shadow as lifting up the truth by Locke’s system of sensational philosophy. Impressions are simple, atomic. They come into the mind, one after another. They cohere in no unity. They are held together by no necessary relation. They are separate, one from the other. If there is no primary, innate faculty; no abiding and indwelling mental activity, that lies behind, and determines and co-ordinates the objects which nature supplies through the senses, converting them into rational, orderly knowledge, then we can never get hold of truth. We are shut up to hopeless ignorance. VI. Berkeley, in order to escape the materialism to which Locke’s philosophy led, accepted his theory of knowledge, but destroyed his outward, material world. In his view, there was no matter, nothing but ideas. The impressions conveyed through the senses into our minds are but reflections of the ideas of God.