Here’s the outline of a paper I´m starting to work on. If anybody has some spare time and wants to take a look, comments are very welcome (sorry for the length)!
Nominalists about the ontological constitution of material objects aim to dispense with both universals and bare particulars and yet provide an economic and compelling account of similarity and individuation.
Resemblance nominalism is the view that only concrete particulars exist, and properties are derivative on similarity classes of such particulars. This view has to deal with the traditional Goodmanian objections based on the possibility of coextension, imperfect community and companionship; it must also explain why the very same object couldn’t have any properties whatsoever (since an object’s belonging to a similarity class appears to be a contingent fact). Rodriguez-Pereyra recently defended resemblance nominalism by endorsing counterpart theory (every object possesses its properties - i.e., partakes in specific similarity classes - necessarily) and realism about possible worlds (the coextension problem is solved if similarity classes also comprise merely possible objects); and proposing a complex notion of resemblance, according to which resemblance holds in various degrees and in an iterative way - between pairs of objects, pairs of pairs of objects etc. (this latter move neutralises the problems of imperfect community and companionship). These are, clearly, non-negligible commitments. An alternative would be to give up the assumption that ordinary objects are the ‘unit of discourse’ and assume that the fundamental building blocks of reality are simple (=belonging to one similarity class) concrete particulars. This would immediately solve the Goodmanian difficulties. However, the problem with the contingency of property-possession remains. If one doesn’t like counterpart theory, it would seem, this problem can only be obviated by going trope-theoretic, that is, by identifying each simple concrete object belonging to only one similarity class with its ‘qualitative content’.
Trope theory, however, has the problem that at least some properties appear dependent on objects rather than constitutive of them (think of colour, or shape properties): with respect to their identity (this table’s hardness, not this hardness, which may or may not compose a table) and their number (since I can tear this white sheet in arbitrarily many pieces, it looks as though there is no fixed number of whiteness tropes in it - the so-called boundary problem). The obvious solution is to endorse a sparse and reductionist account according to which only physically basic, simple properties (e.g., the mass or charge of elementary particles) are genuine tropes. However, this seems to go in the direction of resemblance nominalism, as the trope-theorist attempts to defend the view by making tropes concrete, rather than abstract, particulars.
This may seem circular. However, think about the difference between an elementary particle and its qualitative aspects (mass, charge, spin, colour): do they belong to clearly distinct ontological categories? Or would it be plausible to regard mass etc. as material constituents of a more complex, but equally concrete, particular? A third way emerges, in which the nominalist (thanks to the abovementioned sparse-reductionist approach to properties) takes simple, concrete particulars essentially provided with a qualitative content as fundamental entities. Interestingly, this view was proposed by Sellars already in 1963 (‘Particulars’), where he argues in detail that the property/object distinction can and should be overcome, and proposes an ontology of ‘simple particulars’. Perhaps it would be interesting (for nominalists at least) to examine this Sellarsian option in more detail?