 Bigelow and Pargetter’s (1990) relational account aims to explain both why determinates are similar and why they differ. They see irreducible determinables as insufficient unto this task, on grounds, first, that taking determinables to be irreducibly distinct from but necessarily accompanied by determinates introduces unexplained necessary connections (1990: 118), and second, that determinables alone don’t explain differences between determinates (1990: 54, 58; see also Armstrong 1978b: 113). Bigelow and Pargetter rather suggest that what it is for determinates to “fall under a common determinable” is for the determinates to instantiate second-degree relations of difference from one another, and for pairs of these differences to in turn instantiate third-degree proportions (1990: 58–62).