  This shift to relations as individuating elements has led to the development of a form of PII, based on Quine’s suggestions about discernibility, which allows objects to be ‘weakly’ discernible in relational terms (Saunders 2003a and 2006; for a useful overview see Bigaj 2015a). Consider for example, two fermions in a spherically-symmetric singlet state. The fermions are not only indistinguishable in the above sense but also possess exactly the same set of spatio-temporal properties and relations. However, each enters into the symmetric but irreflexive relation of ‘having opposite direction of each component of spin to …’ on the basis of which they can be said to be ‘weakly discernible’ (for general discussions of different kinds of discernibility see Caulton and Butterfield 2012a; Bigaj 2014; Ketland 2011; Ladyman, Linnebo and Pettigrew 2012). If we extend PII to incorporate such relations, the Principle can, it seems, be made compatible with quantum physics and the individuality of the fermions can be grounded in these irreflexive relations, without having to appeal to anything like primitive thisness. This result has also been extended to bosons (Muller and Saunders 2008; Muller and Seevinck 2009), although some of the details are contentious, in particular with regard to the interpretation of some of the mathematical features that are appealed to in this account (see Bigaj 2015a and 2015b; Caulton 2013; Huggett and Norton 2014; Norton 2015). In addition to such technical issues, there is the further philosophical concern that the appeal to irreflexive relations in order to ground the individuality of the objects which bear such relations involves a circularity: in order to appeal to such relations, one has had to already individuate the particles which are so related and the numerical diversity of the particles has been presupposed by the relation which hence cannot account for it (see French and Krause 2006; Hawley 2006 and 2009). One response to this worry would be to question the underlying assumption that relata must have the relevant ontological priority over relations and adopt some form of structuralist view of objects according to which the relata are eliminable in terms of relations (perhaps ‘emerging’, in some sense as ‘intersections’ of them) or, more mildly perhaps, argue that neither are accorded priority but come as a ‘package’ as it were (for further discussion see French 2014). It has been suggested, for example, that this whole discussion of weak discernibility reveals a category of entity that has received little attention so far, namely that of ‘relationals’: objects that can be discerned by means of relations only (Muller 2011, 2015). I shall return to the structuralist perspective below (but for an alternative, ‘coherentist’ account, see Calosi and Morganti 2018). More generally, however, it has been argued that this whole debate is orthogonal to that over the status of PII since what weak discernibility grounds is merely numerical distinctness, rather than the robust sense of discernibility that PII was originally concerned with (Ladyman and Bigaj 2010). The latter involves some sense of difference over and above numerical distinctness but weakly discernible relations such as ‘having opposite direction of each component of spin to …’ do not provide this. Hence, it is claimed, PII remains violated by quantum mechanics (although see Friebe 2014 where the principle is defended in the context of a specific understanding of quantum entanglement). 