The activity of naming or, in general, of using words may be seen as the human peculiarity of making explicit a process that is quite universal among higher animals, namely, the organization of sensory data. All vertebrates are equipped to superimpose categories of functional equivalence upon stimulus configurations, to classify objects in such a way that a single type of response is given to any one member of a particular stimulus category. The criteria or nature of categorization has to be determined empirically for each species. Frogs may jump to a great variety of flies and also to a specific range of dummy-stimuli, provided the stimuli preserve specifiable characteristics of the ‘real thing’.

Furthermore, most higher animals have a certain capacity for discrimination. They may learn or spontaneously begin to differentiate certain aspects within the first global category, perhaps by having their attention directed to certain details or by sharpening their power of observation. In this differentiation process initial categories may become subdivided and become mutually exclusive, or a number of coexisting general and specific categories or partially overlapping categories may result. Again, the extent of a species’ differentiation capacity is biologically given and must be ascertained empirically for each species. Rats cannot make the same range of distinctions that dogs can make, and the latter are different in this respect from monkeys. The interspecific differences cannot merely be explained by differences in peripheral sensory thresholds. Apparently, a function of higher, central processes is involved that has to do with cognitive organization.

Most primates and probably many species in other mammalian orders have the capacity to relate various categories to one another and thus to respond to relations between things rather than to things themselves; an example is ‘to respond to the largest of any collection of things’. Once more, it is a matter of empirical research to discover the limits of relations that a species is capable of responding to.

In summary, most animals organize the sensory world by a process of categorization, and from this basic mode of organization two further processes derive: differentiation or discrimination, and interrelating of categories or the perception of and tolerance for transformations. In man these organizational activities are usually called concept-formation; but it is clear that there is no formal difference between man’s concept-formation and an animal’s propensity for responding to categories of stimuli. There is, however, a substantive difference. The total possibilities for categorization are clearly not identical across species.