It is perhaps an analytic statement – that is, self-evidently true – that the only way in which members of our species, Homo sapiens, can order their perceptions of the world and the ideas to which they give rise is to produce a classification. There is every reason to believe that this need also applies to other animals and is by no means confined to those animals closely related to man. It is literally vital to any animal that it must have a series of metaphorical compartments in which it places perceived phenomena – food, drink, shelter, danger, own species (sub-divided as parent, sib, rival, sexual partner, etc.). We also know that with greater intelligence, learning ability, and general mental flexibility, the classifications employed by higher animals, although articulated only in our own species, are more complex and subtle than those of lower animals. In humankind our knowledge is ordered in many precise, ranked classifications, although the individual person is unlikely to be aware of the structure of the classification he or she is using.