ABSTRACT
The classification of diseases (a subject rather surprisingly not directly discussed in the Essay, given Locke's medical interests)148 supplies one of the more popular illustrations from which the modern theory of natural kinds sets out. Diseases are certainly 'modes', but the name of a disease will normally be introduced, and then be generally applied, on the basis of repeated experience of a set of symptoms, and on the assumption that on each occurrence they have the same common cause, whether a microbe or an underlying physiological condition. That assumption may come to be thought false, as has perhaps happened in the case of 'rheumatism'. As in biological classification, the name will then normally drop out, or be employed for a newly identified classis. If, however, the name is (like 'rheumatism') retained, but linked rigidly to the old set of symptoms, it becomes a descriptive or practical term like 'headache', which is not the name of a 'natural kind' in the sense of the modern theory. In another kind of case, it may be that a disease
occurs, but without the characteristic symptoms; just as a substance might turn out to be a metal, since it has the fundamental nature of metal, even though, for some special reason, it lacks many of the properties characteristic of metals.
