Space and time therefore form the _principium individuationis_. By what marks, however, may we recognize the identity of an individual at different times and in different places? The identity of inorganic matter depends on the continuity of the mass of atoms which compose it; that of living beings upon the permanent organization of their parts (different bodies are united into _one_ animal by a common life); personal identity consists in the unity of self-consciousness, not in the continuity of bodily existence (which is at once excluded by the change of matter). The identity of the person or the ego must be carefully distinguished from that of substance and of man. It would not be impossible for the person to remain the same in a change of substances, in so far as the different beings (for instance, the souls of Epicurus and Gassendi) participated in the same self-consciousness; and, conversely, for a spirit to appear in two persons by losing the consciousness of its previous existence. Consciousness is the sole condition of the self, or personal identity.--The determinations of space and time are for the most part relations. Our answers to the questions "When?" "How long?" "How large?" denote the distance of one point of time from another (_e.g._, the birth of Christ), the relation of one duration to another (of a revolution of the sun), the relation of one extension to another well-known one taken as a standard. Many apparently positive ideas and words, as young and old, large and small, weak and strong, are in fact relative. They imply merely the relation of a given duration of life, of a given size and strength, to that which has been adopted as a standard for the class of things in question. A man of twenty is called young, but a horse of like age, old; and neither of these measures of time applies to stars or diamonds. Moral relations, which are based on a comparison of man's voluntary actions with one of the three moral laws, will be discussed below.