Knowledge is explained by the principle that elements in the things outside us are perceived by the corresponding elements in ourselves. Like is known by like. The whole body is full of pores and hence respiration takes place over the whole frame. In the organs of sense these pores are specially adapted to receive the effluences which are continually rising from bodies around us; thus perception occurs. In vision, certain particles go forth from the eye to meet similar particles given forth from the object, and the resultant contact constitutes vision. Perception is not merely a passive reflection of external objects.