This truth is by no means new; it lay by implication in the reflections of Descartes; but Berkeley first distinctly enunciated it; while Kant erred by ignoring it. So ancient is it that it was the fundamental principle of the Indian Vedanta, as Sir William Jones points out. In one aspect the world is idea; in the other aspect, the world is will. That which knows all things and is known by none is the subject; and for this subject all exists. But the world as idea consists of two essential and inseparable halves. One half is the object, whose form consists of time and space, and through these of multiplicity; but the other half is the subject, lying not in space and time, for it subsists whole and undivided in every reflecting being. Thus any single individual endowed with the faculty of perception of the object, constitutes the whole world of idea as completely as the millions in existence; but let this single individual vanish, and the whole world as idea would disappear. Each of these halves possesses meaning and existence only in and through the other, appearing with and vanishing with it. Where the object begins the subject ends. One of Kant's great merits is that he discovered that the essential and universal forms of all objects--space, time, causality--lie _a priori_ in our consciousness, for they may be discovered and fully known from a consideration of the subject, without any knowledge of the object. Ideas of perception are distinct from abstract ideas. The former comprehend the whole world of experience; the latter are concepts, and are possessed by man alone amongst all creatures on earth; and the capacity for these, distinguishing him from the lower animals, is called reason. Time and space can each be mentally presented separately from matter, but matter cannot be thought of apart from time and space. The combination of time and space in connection with matter constitutes action, that is, causation. The law of causation arises from change, that is from the fact that at the same part of space there is now one thing and then another, and this succession must be the result of some law of causality, seeing that there must be a determined part of space and a determined part of space for the change. Causality thus combines space with time.