 By contrast, Frege rejects the idea that logic is a purely formal enterprise (MacFarlane 2002, 29; Linnebo 2003, 243). He took logic to have its own unique subject matter, which included not only facts about concepts (concerning negation, subsumption, etc.) and identity, but also facts about relations (e.g., their properties and ancestrals). Frege (1906, 428 [1984, 338]) says: