Suggestion consists in the action of ideas or representations on the activity of the brain in general, and on some of its activities in particular. The terms _idea-force_ and _ideoplasty_ have been employed; but all ideas are at the same time forces and are more or less ideoplastic according to the nature and intensity of the cerebral activity which corresponds to them. Every representation which appears in our consciousness is at the same time a cerebral activity. I will explain by the aid of an example the relation which exists between the play of our _conscious_ ideas and what is incorrectly called our _unconscious_ cerebral activity. For reasons which are too long to explain here, I call _subconscious_ all which is usually called unconscious, because I maintain that there is probably nothing unconscious in our nervous activity, and that what appears to be so is in reality accompanied by an introspection, subordinated like its corresponding activity to the great and clear introspection of the higher brain, which accompanies the concentrated and mobile activity of what we call our attention in the waking state. No doubt, we do not as a rule perceive our subconscious activities, for want of sufficient intensity in their association with the series of aperceptions (states subsequent to attentional activity). But we possess a number of observations, due especially to hypnotism, which allow us to infer by analogy the existence of subordinated introspections corresponding to the cerebral activities which appear to us unconscious. For example, I think of my wife. This idea immediately calls to mind that of a journey that I intend to take with her, and in its turn the idea of the journey recalls that of the trunk I shall use to pack my effects. Almost as rapidly as lightning, the three ideas: (1) my wife; (2) the journey; (3) the trunk, apparently succeed each other in my consciousness. But, according to the old scholasticism, the idea of the journey is awakened by that of my wife, and that of my trunk by that of the journey, which would, therefore, be its "cause." But a little observation soon shows that the succession of our conscious ideas is not so easily explained, for at every moment representations appear which have no logical relation to those which precede them, and cannot be caused by them, nor by immediate sensory perceptions coming from without. At a time when the activity of the brain was not understood, the existence of an essential mind and a free will were assumed, independent of the law of the conservation of energy and of the law of causality, independent therefore of the brain, the activity of which they commanded more or less at their pleasure. This conception is based on ignorance of the facts.