In book three, chapter six of his Essay, Locke discusses the relationship between species and essence in the context of a broader discussion of the names of ideas of substances. In the sixth section of the chapter, Locke appears to claim that real essences are properties of species or sorts, rather than individuals. Interpreting the passage in this way poses two related problems for Locke’s theory. First, if Locke is claiming that the real essence, rather than the nominal essence, is closely related to the species, then this is a direct contradiction of what Locke says about which type of essence is possessed by species elsewhere in the essay. Second, given Locke’s criteria for the demarcation of real from nominal essences earlier in the essay, it is difficult to see how a claim that substantial species have real essences is even comprehensible. I want to consider these problems in more detail and offer an alternative interpretation of the passage in question that attempts to avoid attributing contradictory or unintelligible views to Locke.
The passage at issue is found in III.vi.6. Here Locke is discussing the difference between real and nominal essence in substances, and after defining real essence using his standard definition (the real constitution of a thing which is the foundation of its properties), Locke proceeds with the odd claim that essence “even in this sense, relates to a Sort, and supposes a Species: For being that real Constitution, on which the Properties depend, it necessarily supposes a sort of Things, Properties belonging only to Species, and not to Individuals.” An individual parcel of matter has no essential properties; it is just the arrangement of minute particles and their motion, which can change or be changed by its interactions with other bodies in the world. Therefore, according to Locke, any properties referred to as essential must be so only as part of a species concept, since it is only our general ideas which possess the requisite immutability to have essential properties. A “face value” interpretation of this passage leads us to the conclusion that Locke holds that species, not individuals, possess real essences.
The first problem with the face value interpretation is that the claim that species have real, instead of nominal essences, is in direct contradiction with things Locke says elsewhere in the Essay. For example, in III.iii.11-12 Locke concludes that “General and Universal do not belong to the real existence of Things” but are inventions of the mind for use in organizing perception and “concern signs only”. A general term signifies a class of a sort of thing by “being a sign of an abstract Idea in the mind”; under which particulars are classed which “agree” with the abstract idea. Species are on this account nothing but abstract ideas. Species do not exist in the world. To be a member of a species is nothing more than to have a right to be called the name of that species, a right which is derived from agreement with the abstract idea. This is the type of idea that Locke speaks about a few sections later in III.iii.15-16 when discussing nominal essence, which instead of referring to the real, hidden constitution of things is applied to the “artificial Constitution of Genus and Species.” This is highly suggestive that Locke does not consider species of substances to have real essences (i.e., real constitutions) but rather artificial constitutions and nominal essences.
In addition to these passages suggesting that species have only nominal, and not real, essences, there are passages that indicate that only the nominal essence is used in the formation of species concepts. III.vi.7-8 indicates that the nominal essence is at work in the classification of species. It is impossible to arrange or classify substances by reference to their real essences (which are unknown), so the only means by which to erect species boundaries is by reference to their nominal essences. We class as the same species different particulars whose properties agree with the abstract idea that is signified by the name of the species. Locke also cites as additional evidence that species classifications are based on nominal essences rather than real essences the fact that two objects of the same species can differ from each other in their inessential characters as much as they might differ from another object from another species, without disturbing that classification, even though all properties flow from the same real constitution. A similar point is made by Locke at III.iii.17. So if we are to keep Locke from contradicting himself and also interpret our original passage as indicating that Locke holds that species, not individuals possess real essences, we will owe an explanation as to why species are listed as just the types of things which have nominal essences instead of real essences in the case of substances as well as why only the nominal essence would be useful in species classification.
In addition to the problem of keeping Locke from contradicting himself, there is the problem of making sense of what a real essence of species could possibly be, given Locke’s discussion of the differences between real and nominal essences. For Locke, real essences are related to the real constitutions of things, which are unknown in substances (III.iii.15). In substances, those essences are material; they are the physical arrangements of the parts of substances, from which their properties flow. The conjunction of properties which makes up our idea of a species (i.e., its nominal essence) is distinct from the real essence in substances. Locke claims that the essence of a species is nothing but the abstract idea to which its name is annexed (III.iii.12; III.vi.2) and that no one would assert that the abstract, complex idea of a species is the real essence and source of the properties we observe (III.vi.3).
But if a species is an abstract idea and possesses a nominal essence, what would it mean for the species to have a real essence? In other words, how can an abstract idea have a hidden internal constitution from which the properties of an object flow? Since our complex, abstract ideas are constructs of the mind, Locke believes that we have full access to them—nothing is hidden from us. Also, nothing in Locke indicates he believes species to be material things, and his discussion of the real essences of substances indicates they are material in nature. To attribute real essence to species we would need to resolve this difficulty. In addition, in his discussion of real essences, Locke makes use of causal language to indicate that the real essence is the source of the properties by which our species concepts are formed. Since the species concept (the abstract idea we have of a species) is the nominal essence of a species, by making the species the seat of the real essence Locke would then ensure that the nominal essence would be identical to the real essence in substances, something which Locke explicitly denies (III.iii.18). These considerations ought to make us consider that perhaps reading our original passage as attributing real essences to species is incorrect.
But if Locke is not claiming that species have real essences, what is going on in our original passage? I believe it is possible to interpret III.vi.6 in such a way as to minimize the complications sketched above and still remain faithful to the text and its context. The sections immediately preceding and following the passage in question are occupied with Locke’s attempt to demonstrate that individual parcels of matter do not have essential properties. For example, a piece of iron must possess the ability to be pulled by a magnet if it is to be iron, but there is no necessity for any parcel of matter to be magnetic; in other words, Locke is claiming that it is possible for bodies to have properties other than the ones they have (III.vi.4-5). The properties that individual objects have are then used by the mind to classify objects into kinds and assign nominal essences. The real essences of objects are not made use of in the assignment of species designations to individuals (III.vi.7-8). Essentially, the context in which Locke links species with real essences is an epistemic, rather than a metaphysical one. He is not here claiming that there are real species which exist independently of things that fall under them or of the minds which construct them, but rather that the properties which are referred to as essential to a particular thing are only intelligibly called essences in terms of the sort or species of thing they are taken to be an instance of. He is claiming that the only intelligible way to speak of properties as essential is to do so in the context of species concepts.
I do still want to affirm, however, that Locke intends to tie real essences to our species concepts in this passage. Locke believes that properties which manifest themselves when we observe a substance are caused by the real essence of the substance. The ideas we have of these properties are then combined into the complex idea that forms the nominal essence of the species of the substance; this is the link whereby the real essence is connected to the species. Locke says as much at the end of the section: “as to the real Essences of Substances, we only suppose their Being, without precisely knowing what they are: But that which annexes them still to the Species, is the nominal Essence, of which they are the supposed foundation and cause” (III.vi.6). Basically, species have real essences, but only mediately, through their possession of the nominal essence which is composed of ideas which are caused by the real essences.
This reading of Locke in III.vi.6 allows us to affirm that Locke does tie real essence to species in a deliberate way, as a face value reading indicates. However, by interpreting the claim as one which ties real essence to species through our epistemic or classification practices rather than as positing a real essence directly possessed by a species, we can read Locke without making him contradict himself or claim something incomprehensible.