Metaphysics of natural species (4)

Locke makes a key distinction between real and nominal essence (III, iii, 15, Coste, p. 335). Every individual is certainly endowed with a real internal constitution, the cause of his or her sensible properties. But when we classify individuals - and we do, because we can't help noticing similarities between them - we do so in accordance with certain abstract ideas (III, iii, 17). Even if it is legitimate to speak of a real essence, rationality runs counter to the Aristotelian option. It's not on this that we base our analyses, when we do science, but on the nominal definition. So it doesn't matter that we don't know the actual definition, since it has no explanatory value (III, iii, 17; trans. p. 335). It also means admitting that human understanding will probably never be able to know the underlying constituents of nature (which are insensible). Locke then shows how classifications between species and genera are made, thanks to the concept of nominal essence which is "the work of the understanding" (cf. chap. 6), by means of names, general terms, which are put for different abstract ideas ("sortal" concepts) that have a decisive role in the classifications we make of things (III, vi, 2; trans. p. 353), Locke seeming to draw the conceptualist conclusion, debatable all in all, that, apart from these "sortal" divisions, products of abstraction, there are no classifications (and even less essences) in nature (III, vi, 4), a thesis linked to the key idea that properties are first essential to species before being essential to individuals (III, vi, 6), and that, up to a certain point, real essences are determined in relation to nominal essences. This is followed by a series of arguments, historical and anthropological (linked to the rejection of innate ideas), semantic and epistemological (observation of a non-uniformity in language that would not exist if it were nature that made the classifications), which reinforce anti-aristotelianism and aim to deny any distinction between species in accordance with a natural or internal essence or form (III, vi, 14-26). As noted by M. Ayers [1], Locke is not content to say that ENs are not determined by real essences (epistemological thesis); he believes it possible to conclude that there may be no real divisions in nature at all (strong metaphysical thesis), in accordance with another Locke position (and in keeping with the principle of plenitude and the Great Chain of Beings) that there can be no leaps, but only "degrees", between different things (III, vi, 12, p. 359), and that he has even seen "an animal begotten of a Cat and a Rat. And who had visible marks of these two beasts" (III, vi, 23, p. 364). In a word: even if we can observe great similarities in nature, this in no way allows us to infer that things are classified according to real essences (III, vi, 36-37, p. 365). "The limits of species are established by men". In the case of Gold (III, vi, 50, p. 379), for example, or of Man, and more generally of substances (IV, vi, 4), it is their nominal essence (and therefore the meaning that we give to the terms that designate them by a semantic decision), that allows us to have a form of knowledge (uncertain, incidentally) of the possible limitations of the terms we use to designate them (IV, viii, 4 and 5). Locke does not conclude from our impossibility of knowing the essence of the species to our impossibility of knowing at all. His skepticism is less radical than Hume's, but not far removed.

For Leibniz, on the contrary, every individual substance, in fact, has an essence. "Basically, essence is nothing other than the possibility of what is proposed" (Nouveaux Essais, G, pp. 271-272).

"What we suppose to be possible is expressed by the definition; but this definition is only nominal, when it does not at the same time express the possibility, for then we can doubt whether this definition expresses something real, i.e. possible, until experience comes to our rescue to make us know this reality a posteriori, when the thing is actually in the world; which suffices for the defect of reason, which would make reality known a priori by exposing the cause or possible generation of the thing defined. It does not therefore depend on us to join ideas as we see fit, unless this combination is justified either by reason which shows it possible, or by experience which shows it actual, and consequently possible too."

Leibniz, New Essays, III, 15, Theophilus (emphasis added).

An individual can have "an" essence, which is susceptible to several definitions. But the fact that meanings can vary as we please (Locke) does nothing to answer the question of what the nature of an individual substance consists of (which, for Leibniz, contains a complete concept), nor of what knowledge of this nature consists of. And where Leibniz's Lockean position was hostile to any chiasmus in nature, his conception of modalities, compossibility, possible worlds and divine choice comes directly into play (see Theophilus' reply to Philalethe, §12-13).

[1] Cf. Ayers M., Locke Epistemology & Ontology, 2 vols, London, Routledge, 1991.