There are two elements in sense-perception: a necessary and constant, and a changing and accidental. Space and time are the constant element. They are homogeneous, and always one and the same in quality. They are unities, for there is only one space and one time, and the many spaces and times are only divisions of this oneness. All the differences in space and time are due to the relation and movements of bodies, and are not inherent in space and time themselves. How is this unity and homogeneity of space and time to be explained? By assuming that space and time are original and uniform functions of perception, the forms of perception, the ways of apprehension, the “prehensile organs of our sensibility.” They are the ways in which we synthesize on the lower level of consciousness. If they were given in experience, there is no reason why the several spaces and times should not be intrinsically different, like different bodies with different qualities. However, by conceiving them to be mental syntheses in the level of perception, they explain the universality of the laws of mathematics. They are the colored spectacles that all human beings wear; or, to use another figure, they are the mould into which all sensations are run. Being the unchangeable forms of our sensuous receptivity, they have a validity for the entire compass of perception. They are universal because one experience of space and time is valid for all spaces and times; they are necessary because we cannot think of objects apart from them; they are perceptual syntheses because they increase knowledge. Of course we are unconscious of this perceptual synthesis of the sensory elements in space and time. The process takes place automatically. We can nevertheless analyze the process after it has taken place, and speak of the sensations as the materials of knowledge, and the forms of space and time as the _a priori_ elements. But in actual conscious experience, sensations never come to us in their rawness. They are never turned over to the understanding unless they bear the stamp of space and time. The process of knowledge, therefore, starts with complex material――complex because it has been synthesized below consciousness. In other words, perceptions come into the process of knowledge with two aspects: (1) their permanent and necessary form; and (2) their accidental and changing content.