Sec. 2.  The first use of classification is the better understanding of the facts of Nature (or of any sphere of practice); for understanding consists in perceiving and comprehending the likeness and difference of things, in assimilating and distinguishing them; and, in carrying out this process systematically, new correlations of properties are continually disclosed.  Thus classification is closely analogous to explanation.  Explanation has been shown (chap. xix.  Sec. 5) to consist in the discovery of the laws or causes of changes in Nature; and laws and causes imply similarity, or like changes under like conditions:  in the same way classification consists in the discovery of resemblances in the things that undergo change.  We may say (subject to subsequent qualifications) that Explanation deals with Nature in its dynamic, Classification in its static aspect.  In both cases we have a feeling of relief.  When the cause of any event is pointed out, or an object is assigned its place in a system of classes, the gaping wonder, or confusion, or perplexity, occasioned by an unintelligible thing, or by a multitude of such things, is dissipated.  Some people are more than others susceptible of this pleasure and fastidious about its purity.

A second use of classification is to aid the memory.  It strengthens memory, because one of the conditions of our recollecting things is, that they resemble what we last thought of; so that to be accustomed to study and think of things in classes must greatly facilitate recollection.  But, besides this, a classification enables us easily to run over all the contrasted and related things that we want to think of.  Explanation and classification both tend to rationalise the memory, and to organise the mind in correspondence with Nature.

Every one knows how a poor mind is always repeating itself, going by rote through the same train of words, ideas, actions; and that such a mind is neither interesting nor practical.  It is not practical, because the circumstances of life are rarely exactly repeated, so that for a present purpose it is rarely enough to remember only one former case; we need several, that by comparing (perhaps automatically) their resemblances and differences with the one before us, we may select a course of action, or a principle, or a parallel, suited to our immediate needs.  Greater fertility and flexibility of thought seem naturally to result from the practice of explanation and classification.  But it must be honestly added, that the result depends upon the spirit in which such study is carried on; for if we are too fond of finality, too eager to believe that we have already attained a greater precision and comprehension than are in fact attainable, nothing can be more petrific than ‘science,’ and our last state may be worse than the first.  Of this, students of Logic have often furnished examples.

Sec. 3.  Classification may be either Deductive or Inductive; that is to say, in the formation of classes, as in the proof of propositions, we may, on the whole, proceed from the more to the less, or from the less to the more general; not that these two processes are entirely independent.