Freewill could not separate the Constituents.

For if, as one of the Heretics says, Purity and Foulness were mixed together, it is not Freewill that would be required to separate the good Will from the evil Will, but a strainer to [xxii] separate the pure from the foul. For in the case of things that are literally mixed together, a separating hand is required to separate them like the skilful hand which separates with a fire the dross from the silver, and separates with a strainer the pure from the foul.