He may hold, in the first place, the double aspect theory pure and simple, according to which all developments of mind are merely dependent concomitants of the development of nerve ramifications, with consequent growths of nerve-cells, ganglions, and the more considerable nerve plexuses, culminating in the growth of a brain. This evolution of a nervous and cerebral system he may hold to be wholly due to the action of molecular and other motions upon a mass of colloid substances of such a constitution as to be fittest, under the action of these external stimuli, to form lines for the transmission of motions and for the discharge of these motions into certain otherwise formed contractile structures called muscles. He will consider that they eventually acquire a power of retaining these motions, so that the effect of all the motions thus caused is not immediate but deferred. And since all motions received are not immediately concerned with the welfare of the organism, he may suppose that separate masses of nervous matter are produced, in which these motions are stored in an organised form, related indirectly rather than directly to the motor apparatus. According to this theory the whole system of determining causes is purely physical. In the simple organisms the response of muscular action to incident motions is quick, direct, and unhesitating. Such action is called reflex or automatic, and is as unconscious as chemical activity. But when the system becomes more complex, when nerves cross each other, when cells and junctions are formed, and more particularly when the storages of motions are formed, as just referred to; then compoundings and recompoundings of nervous motions take place, and, according to the strength of the various currents, to the facility of discharge, and to various physical local or general conditions, the action becomes slower and more hesitating. Under these circumstances, it is held that the nervous system becomes conscious. A double aspect then arises, and the actions which thereafter take place may be described either in terms of the relations of the various molecular motions in the nervous and cerebral systems, or in terms of feeling; but all the same the latter is merely the secondary aspect of series of changes altogether determined by the motions and structure of the former. On this theory memory is the revived motion of a nerve structure; feeling is a consciousness of interaction between different nerve motions; trains of thought are the reverberations of great varieties of motions throughout the system and brain; consciousness resulting from the mingling of the nerve currents and the consequent conflict and retardation of effects. The element of mystery here lies in the secondary or subjective aspect, but it is placed strictly without the line of deduction and is a merely unexplained accompaniment of a series of changes otherwise fully accounted for.