To reduce the essentials of abstraction and generalisation to the exclusive use of the word (or sign) as is customary, is an error that can only be explained by the time-honored neglect of the function of the _unconscious_ in psychology. The sign is no more than an instrument of simplification, an abbreviated formula. When the mind works with the aid of concepts, the co-operation of two factors, the one conscious, the other unconscious or subconscious, is required, in order that its labor may be legitimate and fruitful: on the one hand, we have words or signs, accompanied sometimes by a vague representation; on the other hand, a latent, potential, organised knowledge. We endeavored above (Ch. IV.) to show how this _couple_ forms and fixes itself. The mechanism is invariably the same, without exception. Whether we keep up a trivial conversation by means of the abstract terms which compose our languages, or whether we ascend to the highest generalisations, there is in the mental state no more than a difference in degree; there is no difference in nature. Beneath the words that are the clear factors, exists the dumb travail, the vague invocation, of the organised experience that gives life to them. Without this unconscious factor which may, often does, become conscious, there is nought but illusion. When we induct, deduct, traverse a long series of abstractions to demonstrate or to discover, the useful work consists in new relations which establish themselves in our organised potential knowledge; words are no more than the instruments that commence the task, facilitate and mark its phases. When the mind is grappling with the highest abstractions, and climbs from height to height, what preserves it from catastrophe, and guarantees against error, is the quantity and quality of the unconscious material stored up beneath the words. The entomologist who at first sight, and immediately, classifies one insect among millions of species, acts in virtue of his long experience, impressed firmly in his memory with salient characteristics: he proceeds from the sensory data to the name. In the inverse operation, when he merely enunciates the name, all this acquired knowledge is the substrate. The existence of these conscious-unconscious couples is, so to speak, a rule in psychology: general ideas are but a particular case, perhaps the least well-known: hence we previously likened them (Ch. IV.) to _mental habits_.