Conscious intellection is activity of the gray surface of the brain, a tissue consisting of countless nerve-cells united by nerve-fibres. In this tissue the nerves, both of the external bodily surface and of the internal organs, terminate. When one of these nerves is excited (the nerve of vision by a ray of light, a nerve in the skin by contact, an organic nerve by internal chemical action, etc.), it at once conveys the excitement to the nerve-cell in the cerebral cortex in which it debouches. This cell undergoes in consequence chemical changes, which, in a healthy condition of the organism, are in direct relation to the strength of the stimulus. The nerve-cell, which is immediately affected by the stimulus conveyed to it by the conducting nerve, propagates in its turn the stimulus received to all the neighbouring cells with which it is connected by fibrous processes. The disturbance spreads itself on all sides, like a wave-circle that is caused by any object thrown into water, and subsides gradually exactly as does the wave--more quickly or more slowly, with greater or less diffusion, as the stimulus that caused it has been stronger or weaker. Every stimulus which reaches a place on the cerebral cortex results in a rush of blood to that spot,[68] by means of which nutriment is conveyed to it. The brain-cells decompose these substances, and transmute the stored-up energy in them into other forms of energy, namely, into ideas and motor impulses.[69] How an idea is formed out of the decomposition of tissues, how a chemical process is metamorphosed into consciousness, nobody knows; but the fact that conscious ideas are connected with the process of decomposition of tissues in the stimulated brain-cells is not a matter of doubt.[70] In addition to the fundamental property in the nerve-cells of responding to a stimulus produced by chemical action, they have also the capacity of preserving an image of the strength and character of this stimulus. To put it popularly, the cell is able to remember its impressions. If now a new, although it may be a weaker, disturbance reach this cell, it rouses in it an image of similar stimuli which had previously reached it, and this memory-image strengthens the new stimulus, making it more distinct and more intelligible to consciousness. If the cell could not remember, consciousness would be ever incapable of interpreting its impressions, and could never succeed in attaining to a presentation of the outer world. Particular direct stimuli would certainly be perceived, but they would remain without connection or import, since they are by themselves, and without the assistance of earlier impressions, inadequate to lead to knowledge. Memory is therefore the first condition of normal brain activity.