 If properties are conceived of as individuals—otherwise known as tropes, property-instances or individualised forms—then there cannot be a problem about the distinctness of exactly similar bundles, for the difference is built into the identity of the elements of the bundle, as it is not if the bundle is made of universals. The problem for this version of the bundle theory seems to be that it is difficult to individuate or distinguish tropes in a way that makes them suitable to be individuals—in one sense, the substances—from which objects are made (Armstrong 1989: 115ff). For example, if an object has a size, a mass, and a motion, are the size and the mass different tropes and is it not strange to think of the size alone as a genuine particular suited to be treated as an ‘atomic’ component of the whole? One way out of this problem may be to resort to the notion of a master property that was invoked in the discussion of Locke on substratum  (section 2.5.1);  that is, to the notion of one property of which all the others are modes, as visual shape is a mode of colour, or extension is the master property of matter for Descartes. In the Newtonian model, solidity might fill this role, for shape is merely the outline of the solidity which constitutes the heart of the object. In a sense, theories of this kind are not ordinary bundle theories, for one property is chosen to fill the role of substratum, because the other properties inhere in, or are modes of, it. Whether anything could perform this role from amongst the properties that are basic to modern, post-Newtonian physics—properties such as charm, energy, spin and mass—is not easy to judge.