And if the cause manifests an operation sufficient in itself, the co-operating cause indicates assistance and service along with the other. If, accordingly, it effects nothing, it will not be called even a co-operating cause; and if it does effect something, it is wholly the cause of this, that is, of what is produced by it. That is, then, a co-operating cause, which being present, the effect was produced—the visible visibly, and the occult invisibly.