6. EFFICACIOUS GRACE AND MERELY SUFFICIENT GRACE.—By efficacious grace (_gratia efficax_) we understand that divine assistance which with infallible certainty includes the free salutary act. Whether the certainty of its operation results from the physical nature of this particular grace, or from God’s infallible foreknowledge (_scientia media_), is a question in dispute between Thomists and Molinists.(100) Merely sufficient grace (_gratia mere sufficiens_) is that divine assistance whereby God communicates to the human will full power to perform a salutary act (_posse_) but not the action itself (_agere_). The division of grace into efficacious and merely sufficient is not identical with that into prevenient and coöperating. Coöperating grace does not _ex vi notionis_ include with infallible certainty the salutary act. It may indeed be efficacious, but in matter of fact frequently fails to attain its object because the will offers resistance. a) The existence of efficacious graces is as certain as that there is a Heaven filled with Saints. God would be neither omnipotent nor infinitely wise if all His graces were frustrated by the free-will of man. St. Augustine repeatedly expresses his belief in the existence of efficacious graces. Thus he writes in his treatise on Grace and Free-Will: “It is certain that we act whenever we set to work; but it is He [God] who causes us to act, by giving thoroughly efficacious powers to the will.”(101) And in another treatise: “[Adam] had received the ability (_posse_) if he would [_gratia sufficiens_], but he had not the will to exercise that ability [_gratia efficax_]; for if he had possessed that will, he would have persevered.”(102) b) Before demonstrating the existence of sufficient grace it is necessary, in view of certain heretical errors, carefully to define the term. α) Actual grace may be regarded either in its intrinsic energy or power (_virtus_, _potestas agendi_) or in its extrinsic efficacy (_efficientia_, _efficacitas_). All graces are efficacious considered in their intrinsic energy, because all confer the physical and moral power necessary to perform the salutary act for the sake of which they are bestowed. From this point of view, therefore, and _in actu primo_, there is no real but a purely logical distinction between efficacious and merely sufficient grace. If we look to the final result, however, we find that this differs according as the will either freely coöperates with grace or refuses its coöperation. If the will coöperates, grace becomes truly efficacious; if the will resists, grace remains “merely sufficient.” In other words, merely sufficient grace confers full power to act, but is rendered ineffective by the resistance of the will. The inefficacy of merely sufficient grace, therefore, is owing to the resistance of the will and not to any lack of intrinsic power. This is a truth to which all Catholic systems of grace must conform.