 On the other hand, some Locke scholars interpret the concepts of ‘substance’ (in the sense of pure substance in general, or ‘substratum’) and ‘real essence’ as both intensionally and extensionally distinct because they play very different philosophical roles. For example, according to Nicholas Jolley, Locke employs the concept of a real essence when he is addressing issues of scientific explanation and he appeals to substratum when he is discussing the general concept of what it is to be a thing (as opposed to a property or mode) (see Jolley 1999, pp. 70–78).