Commencing thus, without a proper preliminary analysis, we are naturally somewhat at a loss how to present, in a satisfactory manner, those fundamental processes of thought out of which science ultimately originates.  Perhaps our argument may be best initiated by the proposition, that all intelligent action whatever depends upon the discerning of distinctions among surrounding things.  The condition under which only it is possible for any creature to obtain food and avoid danger is, that it shall be differently affected by different objects—­that it shall be led to act in one way by one object, and in another way by another.  In the lower orders of creatures this condition is fulfilled by means of an apparatus which acts automatically.  In the higher orders the actions are partly automatic, partly conscious.  And in man they are almost wholly conscious.

Throughout, however, there must necessarily exist a certain classification of things according to their properties—­a classification which is either organically registered in the system, as in the inferior creation, or is formed by experience, as in ourselves.  And it may be further remarked, that the extent to which this classification is carried, roughly indicates the height of intelligence—­that while the lowest organisms are able to do little more than discriminate organic from inorganic matter; while the generality of animals carry their classifications no further than to a limited number of plants or creatures serving for food, a limited number of beasts of prey, and a limited number of places and materials; the most degraded of the human race possess a knowledge of the distinctive natures of a great variety of substances, plants, animals, tools, persons, etc., not only as classes but as individuals.

What now is the mental process by which classification is effected?  Manifestly it is a recognition of the likeness or unlikeness of things, either in respect of their sizes, colours, forms, weights, textures, tastes, etc., or in respect of their modes of action.  By some special mark, sound, or motion, the savage identifies a certain four-legged creature he sees, as one that is good for food, and to be caught in a particular way; or as one that is dangerous; and acts accordingly.  He has classed together all the creatures that are alike in this particular.  And manifestly in choosing the wood out of which to form his bow, the plant with which to poison his arrows, the bone from which to make his fish-hooks, he identifies them through their chief sensible properties as belonging to the general classes, wood, plant, and bone, but distinguishes them as belonging to sub-classes by virtue of certain properties in which they are unlike the rest of the general classes they belong to; and so forms genera and species.