Despite much scholarly effort, little is known about Roscelin of Compiègne. The only work that we can safely attribute to him is a letter he sent to Peter Abelard around 1119–1120. In this ill-tempered piece of writing Roscelin sets out to distinguish his position on the Trinity from that which Abelard was developing in his Theologia Summi Boni.

The problem for Roscelin and Abelard is to give an account of the distinction between the persons of the Trinity compatible with the unity of God. Roscelin notes that he has to navigate here between two heresies: Sabellianism, requiring such a unity in the singular substance of God that the distinction between the persons can be only verbal, and Ariansm, which distinguishes the persons as greater and lesser so as to constitute three distinct gods. Roscelin, in effect, accused his former student of Sabellianism, and so contributed to...