The typical and abstract idea of white has not merely a nominal value, as it is asserted in some schools of thought, for an empty term could express no idea, whereas this idea is perfectly clear. Neither is it a real thing, but rather an ideal reality, not a pure abstraction of the spirit, extracted, so to speak, from the material substance. The conception of whiteness formed by the comparative judgment is limited by the perception of the concrete, external fact perceived as one special quality among all other qualities in nature, and it is therefore a physiological fact of inward consciousness. In the abstract idea of white or whiteness we do not only picture to ourselves a quality common to many things, but by this term, and by the idea which corresponds to it, the same sensation is actually present to our inward intuition, or the same quality of the sensation which was previously generated by our external senses in a concrete form. Although, therefore, the idea is generic, the sensation itself is represented to the mind in the form of a concrete perception. It is not concrete in the sense of belonging to a special object or definite form, as it is presented to the outward perception, but only so far as there is actually an inward and physiological sensation of whiteness, which the word recalls to the memory. There can be no mental confusion with the quality of red, or of any colour, when I speak or think of what is white. When I speak or think of any object as white, I and others perfectly understand what is meant, and a representation of this quality is instantly formed in our minds, in the generic type which was gradually constituted by primitive man by the combination of numerous special sensations, obvious to the sight, and subsequently expressed in speech. In order that the word which corresponds to the quality may have a given sense, it is necessary to perceive the form of the concrete sensation which gave rise to it; for although the representation is indefinite or generic, that is, not obvious to the external senses, yet it is not physiologically distinct from the sensation of the quality described; the perception of that quality is present by the aid of memory to the inner consciousness. It is therefore evident that the physiological elements of consciousness are actually contained in so-called abstract ideas, although it is sometimes asserted that they are purely spiritual and intellectual acts, remote from every physiological process of fact and sense. An actual physiological fact (colour in this instance) corresponds to the idea in the nervous centres, and reproduces the sensation due to the perception of special objects, whose physical quality of whiteness we have perceived, and this sensation makes part of the abstract, or rather indefinite conception.