Purposes are either ([alpha]) special or practical, as in gardening or hunting, or ([beta]) general or scientific, as in Botany or Zoology.  The scientific purpose is merely knowledge; it may indeed subserve all particular or practical ends, but has no other end than knowledge directly in view.  And whilst, even for knowledge, different classifications may be suitable for different lines of inquiry, in Botany and Zoology the Morphological Classification is that which gives the most general and comprehensive knowledge (see Huxley, On the Classification of Animals, ch. 1).  Most of what a logician says about classification is applicable to the practical kind; but the scientific (often called ’Natural Classification’), as the most thorough and comprehensive, is what he keeps most constantly before him.

Scientific classification comes late in human history, and at first works over earlier classifications which have been made by the growth of intelligence, of language, and of the practical arts.  Even in the distinctions recognised by animals, may be traced the grounds of classification:  a cat does not confound a dog with one of its own species, nor water with milk, nor cabbage with fish.  But it is in the development of language that the progress of instinctive classification may best be seen.  The use of general names implies the recognition of classes of things corresponding to them, which form their denotation, and whose resembling qualities, so far as recognised, form their connotation; and such names are of many degrees of generality.  The use of abstract names shows that the objects classed have also been analysed, and that their resembling qualities have been recognised amidst diverse groups of qualities.

Of the classes marked by popular language it is worth while to distinguish two sorts (cf. chap. xix.  Sec. 4):  Kinds, and those having but few points of agreement.

But the popular classifications, made by language and the primitive arts, are very imperfect.  They omit innumerable things which have not been found useful or noxious, or have been inconspicuous, or have not happened to occur in the region inhabited by those who speak a particular language; and even things recognised and named may have been very superficially examined, and therefore wrongly classed, as when a whale or porpoise is called a fish, or a slowworm is confounded with snakes.  A scientific classification, on the other hand, aims at the utmost comprehensiveness, ransacking the whole world from the depths of the earth to the remotest star for new objects, and scrutinising everything with the aid of crucible and dissecting knife, microscope and spectroscope, to find the qualities and constitution of everything, in order that it may be classed among those things with which it has most in common and distinguished from those other things from which it differs.  A scientific classification continually grows more comprehensive, more discriminative, more definitely and systematically coherent.  Hence the uses of classification may be easily perceived.