“Alia est iniquitas nostra, alia calcanei nostri, in quo Adam dente serpentis est vulneratus et obnoxiam hereditatem successionis humanæ suo vulnere dereliquit, ut omnes illo vulnere claudicemus.” St. Aug. Exp. Psal. xlviii. 6, and St. Ambrose, Enar. in Ps. xlviii. 9: “Unde reor uniquitatem calcanei magis lubricum deliquendi quam reatum aliquem nostri esse delicti.” This lubricum delinquendi, the wound of Adam’s heel, seems to have been understood of concupiscence, which has the nature of sin, and is called sin by St. Paul.