From Smallpox to Nominal Essences: How Locke changed his mind about natural kinds

Martin Lenz

University of Groningen


As is well-known, Locke endorsed a distinction between real and nominal essences. Since the real essences of things are unknowable to us, our categorizations are based on nominal essences, i.e. conventional classifications that are adapted to our needs. While there have been numerous debates about the characteristics of this distinction (see Atherton 2007), very little is known about its development and motivation. Thus, it is still an open question whether Locke actually was a some sort of essentialist (Anstey 2011), an anti-essentialist (Ayers 1991) or agnostic about their existence (Lenz 2010). The lack of interest in the actual motivation of the distinction is methodologically odd, since the circumstances of its invention and its maturation should tell us more about its actual meaning.

My paper aims at filling this gap by addressing the development and role of the distinction from Locke’s earliest works to the Essay. What is hardly recognized is that Locke started out as an unwavering essentialist. I shall argue that the distinction was, initially at least, not owing to nominalism or worries about classification. Rather it was motivated by Locke’s methodological reflections on natural philosophy, more precisely in medicine, as testified by the so-called Smallpox-Fragment (1670) that predominantly owes to the collaboration with the “English Hippocrates” Thomas Sydenham. In a nutshell, Locke’s view of essences is restricted by what we today might call ‘pragmatism’. It was gradually fleshed out by his methodological stance in natural philosophy as well as his views on the social determination of linguistic meaning. I will begin by sketching Locke’s commitment to essences and the ‘principle of unity in nature’ in his Questions on the law of nature (1664). Against this background, I will introduce his methodological considerations in his writings on medicine and compare how they fare in relation to the mature distinction between real and nominal essences in the Essay (1689). By way of conclusion, I will discuss the ensuing pragmatist constraints on essentialism.

In a careful look at the specific problems Locke tackled in collaboration with Sydenham, then, I hope to show that Locke did not simply criticize the essentialism of the scholastics, but tried to establish a refined notion of essences that captures the various natural and normative features traditionally associated with essences, namely both the definitional aspects of kind-relation and intelligibility as well as the modal aspects of necessity and potentiality, in a new way that does justice to our knowledge about nature as well as the pragmatic requirements that constrain it. If the suggested re-reading of Locke’s distinction is correct, the crucial point is not so much his stance on the debate between essentialists and anti-essentialists. His main contribution should rather be seen as a transformation of the notion of essence.