 We may distinguish between weak conventionalism and strong conventionalism. Weak conventionalism asserts that our actual classifications are not, or are very unlikely to be, natural. The principal ground for weak conventionalism is scepticism about the ability of science to uncover the natural principles of classification. We may attribute this reason for conventionalism to Locke (1690: III). It appears (although this is a matter of debate) that Locke thought that if there were real essences of species, these would be found in the shared imperceptible micro-structural properties of things, and that there may be such genuinely natural species but that science was ill-equipped to do better than speculate about them. (Locke also held, employing Criterion 6 above, that there would be ‘gaps’ or ‘chasms’ between genuine species but that observation showed only smooth gradations between the kinds we actually employ, which therefore must be conventional.)