1.7: The "Essence" Criterion

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John Locke
John Locke
(1632-1704)

(Continued from the previous page)

John Ray (1627–1705) seems to have been among the first to offer a definition of species intended for naturalists (as opposed to the ancient definition used by logicians). In his Historia Plantarum (1693), Ray states that

To begin an inventory and proper classification of plants, we need to determine some criterion for distinguishing species. After long and careful consideration of this matter, nothing better has come to mind than those distinctions passed from one generation to another through seed. For whatever traits arise in an individual or in the seed of a species of plant, are accidental and not of the kind that distinguish species. ... For species preserve their distinctive traits forever; one species does not arise from another, nor vice versa.

Of course, there is an obvious difficulty with his definition. Such a rule would result in a huge number of types being treated as species (many cases are known of two types differing with respect to some extremely minor characteristic and yet breeding true for that trait when they are mated only with other individuals of the same type).

Ray's definition represented an effort to provide a working criterion that would allow the classification of plants and animals according to their "specific essences." However, Ray's contemporary, the philosopher John Locke, roundly rejected the idea that systems of classification could be based on "essences" -- though in doing so he constituted a distinct minority. In An Essay Concerning Human Understanding (1690) he tried to show the inadequacies of scholastic doctrine. There he speaks of what was then the

usual supposition, that there are certain precise essences or forms of things, whereby all the individuals existing are, by nature distinguished into species.

At that time, the term species was applied to all types of things, for example, to the various types of minerals. Locke admitted it is possible to classify things into categories on the basis of their observed characteristics. But, while his contemporaries did call those categories "species," he thought they were mistaken to suppose such categories were distinguished by, and based on, "essences" just because the word species was used to refer to them:

It is very true every substance that exists has its peculiar constitution, whereon depend those sensible [i.e., perceptible] qualities and powers we observe in it; but the ranking of things into species (which is nothing but sorting them under several titles) is done by us according to the ideas that we have of them: which, though sufficient to distinguish them by names, so that we may be able to discourse of them when we have them not present before us; yet if we suppose it to be done by their real internal constitutions and that things existing are distinguished by nature into species, by real essences, according as we distinguish them into species by names, we shall be liable to great mistakes.

In point of fact, many make the same sort of mistake even today. That is, when they see that a population is treated as a species, they are likely to assume it has the characteristics they personally associate with the name species. For example, they might assume the population does not interbreed with other such populations. Nevertheless, many populations treated as species do in fact interbreed, often extensively, with other populations treated as species (see Section 2.4). So, as Locke points out, thinking in this way can be a great mistake.

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