Full Idea
Aristotle categorised relations as accidents - Socrates's whiteness, the sphericity of this ball - entities dependent on substances. Relations are not substances, so they must be, if anything at all, accidents.
Gist of Idea
Aristotle said relations are not substances, so (if they exist) they must be accidents
Source
report of Aristotle (Categories [c.331 BCE], §7) by John Heil - Relations 'Historical'
A Reaction
Heil says this thought encouraged anti-realist views of relations, which became the norm until Russell.
Book Reference
'Routledge Companion to Metaphysics', ed/tr. Le Poidevin/Simons etc [Routledge 2012], p.312