El extenso tema de la Clasificación se ha pospuesto para una parte posterior de este tratado, porque entraña cuestiones de dificultad y no parecía encajar de manera natural en un lugar anterior.
Pero no debe suponerse que, al abordar ahora formalmente el tema, estemos considerando por primera vez la noción de clasificación. Toda inferencia lógica implica clasificación, la cual es de hecho el acompañamiento necesario de la acción del juicio. Es imposible detectar similitud entre objetos sin unirlos por ello en el pensamiento y formar una clase incipiente.
Tampoco podemos otorgar un nombre común a los objetos sin implicar la existencia de una clase. Todo nombre común es el nombre de una clase, y todo nombre de una clase es un nombre común. Es evidente también que hablar de una noción general o concepto no es sino otra manera de hablar de una clase.
El uso nos lleva a emplear la palabra clasificación en algunos casos y en otros no. Se dice que formamos la noción general paralelogramo cuando consideramos que un número infinito de posibles figuras rectilíneas de cuatro lados se asemejan entre sí en la propiedad común de poseer lados paralelos. Se diría que formamos una clase, Trilobite, cuando reunimos en un museo a una serie de especímenes que se asemejan entre sí en ciertos caracteres definidos. Pero la naturaleza lógica de la operación es la misma en ambos casos. Formamos una clase de figuras llamadas paralelogramos y formamos una noción general de trilobites.
Science, it was said at the outset, is the detection of identify, and classification is the placing together, either in thought or in actual proximity of space, those objects between which identity has been detected. Accordingly, the value of classification is co-extensive with the value of science and general reasoning. Whenever we form a class we reduce multiplicity to unity, and detect, as Plato said, the one in the many. The result of such classification is to yield generalised knowledge, as distinguished from the direct and sensuous knowledge of particular facts. Of every class, so far as it is correctly formed, the principle of substitution is true, and whatever we know of one object in a class we know of the other objects, so far as identity has been detected between them. The facilitation and abbreviation of mental labour is at the bottom of all mental progress. The reasoning faculties of Newton were not different in nature from those of a ploughman; the difference lay in the extent to which they were exerted, and the number of facts which could be treated. Every thinking being generalises more or less, but it is the depth and extent of his generalisations which distinguish the philosopher. Now it is the exertion of the classifying and generalising powers which enables the intellect of man to cope in some degree with the infinite number of natural phenomena. In the chapters upon combinations and permutations it was made evident, that from a few elementary differences immense numbers of combinations can be produced. The process of classification enables us to resolve these combinations, and refer each one to its place according to one or other of the elementary circumstances out of which it was produced. We restore nature to the simple conditions out of which its endless variety was developed. As Professor Bowen has said,1 “The first necessity which is imposed upon us by the constitution of the mind itself, is to break up the infinite wealth of Nature into groups and classes of things, with reference to their resemblances and affinities, and thus to enlarge the grasp of our mental faculties, even at the expense of sacrificing the minuteness of information which can be acquired only by studying objects in detail. The first efforts in the pursuit of knowledge, then, must be directed to the business of classification. Perhaps it