It was customary in earlier psychology, as it still is in all apologetic psychology, to regard the soul as a unified, immaterial, indivisible and therefore indestructible substance, as a monad, which, as a unity without parts, superior to its own capacities and the changes of its states, is at all times one and the same subject. Many attempts have been made since the time of Plotinus to accumulate proofs of this substantial unity. We may leave this question untouched here, and need not even inquire whether these definitions are not themselves things of the external world employed as images and analogies and pushed too far. But there are three factors which may be established in regard to the psychical in spite of all naturalistic opposition; and those who have attempted to find proofs for the traditional idea we have noted, have usually really had these three in mind, and quite rightly so: they are, self-consciousness, the unity of consciousness, and the consciousness of the ego.