  In his commentary on the Categories, Boethius highlights an interesting feature of Aristotle’s discussion of relations.  Whenever Aristotle refers to the category of relations, he always does so in the plural (‘things toward something’), thereby departing from his usual practice of referring to categories in the singular (‘substance’, ‘quantity’, ‘quality’, etc.). This certainly reinforces the paradigmatic analysis of relational situations, which requires relations to come in pairs. But Boethius thinks there is a further reason why Aristotle refers to the category in this way—namely, because its members are unique in that they cannot be understood to exist by themselves: