 In fact there are three issues concerning the material underpinning of things that Locke regards as mysterious, and he seems to move indifferently from one to the other. First, there is organisation of the minute parts of particular kinds of objects, which is responsible for the manifest properties of those objects, and which he thinks will always fall beyond our knowledge. Second is the mystery of what holds the minute parts together: the problem of explaining attraction in a system that only understands influence by impact (xxiii 23). Third, is that idea of substance as a bare substratum, which is “a supposed, I know not what, to support those ideas we call accidents” (xxiii 15).