For example, sometimes we cannot recall in the morning a melody which the night before ran in our head till we were tired of it. The cases in which a thought or a picture of the fancy suddenly came into our mind without any conscious occasion seem to afford an exception to what has been said. Yet this is for the most part an illusion, which rests on the fact that the occasion was so trifling and the thought itself so vivid and interesting, that the former is instantly driven out of consciousness. Yet sometimes the cause of such an instantaneous appearance of an idea may be an internal physical impression either of the parts of the brain on each other or of the organic nervous system upon the brain. In general our internal process of thought is in reality not so simple as the theory of it; for here it is involved in many ways. To make the matter clear to our imagination, let us compare our consciousness to a sheet of water of some depth. Then the distinctly conscious thoughts are merely the surface; while, on the other hand, the indistinct thoughts, the feelings, the after sensation of perceptions and of experience generally, mingled with the special disposition of our own will, which is the kernel of our being, is the mass of the water. Now the mass of the whole consciousness is more or less, in proportion to the intellectual activity, in constant motion, and what rise to the surface, in consequence of this, are the clear pictures of the fancy or the distinct, conscious thoughts expressed in words and the resolves of the will. The whole process of our thought and purpose seldom lies on the surface, that is, consists in a combination of distinctly thought judgments; although we strive against this in order that we may be able to explain our thought to ourselves and others. But ordinarily it is in the obscure depths of the mind that the rumination of the materials received from without takes place, through which they are worked up into thoughts; and it goes on almost as unconsciously as the conversion of nourishment into the humours and substance of the body. Hence it is that we can often give no account of the origin of our deepest thoughts. They are the birth of our mysterious inner life. Judgments, thoughts, purposes, rise from out that deep unexpectedly and to our own surprise. A letter brings us unlooked-for and important news, in consequence of which our thoughts and motives are disordered; we get rid of the matter for the present, and think no more about it; but next day, or on the third or fourth day after, the whole situation sometimes stands distinctly before us, with what we have to do in the circumstances. Consciousness is the mere surface of our mind, of which, as of the earth, we do not know the inside, but only the crust.