The proofs from the philosophers that I collected above, I collected not from their own writings (few of which I'm familiar with) but rather from the books of the holy fathers.Likewise I owe my knowledge of this sentence not to Abelard's works, but to Domenico Comparetti, Vergil in the Middle Ages, tr. E.F.M. Benecke (1895; rpt. Princeton: Princeton University Press, 1997), p. 197, n. 3.
Quae enim superius ex philosophis collegi testimonia, non ex eorum scriptis, quorum pauca novi, immo ex libris Sanctorum Patrum collegi.
Although I'm not a professor, I usually do try to follow "Common Rules of the Professors of Higher Faculties," § 8, Ratio Studiorum (tr. Allan P. Farrell):
It scarcely becomes the dignity of a professor to cite an authority whose works he himself has not read.