John Heil 
Relations [PDF ebook] 

Historically, philosophical discussions of relations have featured chiefly as afterthoughts, loose ends to be addressed only after coming to terms with more important and pressing metaphysical issues. F. H. Bradley stands out as an exception. Understanding Bradley’s views on relations and their significance today requires an appreciation of the alternatives, which in turn requires an understanding of how relations have traditionally been classified and how philosophers have struggled to capture their nature and their ontological standing. Positions on these topics range from the rejection of relations altogether, to their being awarded the status as grounds for everything else, to various intermediary positions along this spectrum. Love them, hate them, or merely tolerate them, no philosopher engaged in ontologically serious metaphysics can afford to ignore relations.