Anti-pelagian Writings by St. Augustine
Chapter III.--(5.) The Fifth Breviate.
V. |We must again,| he says, |inquire whether a man ought to be without sin. Beyond doubt he ought. If he ought, he is able; if he is not able, then he ought not. Now if a man ought not to be without sin, it follows that he ought to be with sin, -- and then it ceases to be sin at all, if it is determined that it is owed. Or if it is absurd to say this, we are obliged to confess that man ought to be without sin; and it is clear that his obligation is not more than his ability.| We frame our answer with the same illustration that we employed in our previous reply. When we see a lame man who has the opportunity of being cured of his lameness, we of course have a right to say: |That man ought not to be lame; and if he ought, he is able.| And yet whenever he wishes he is not immediately able; but only after he has been cured by the application of the remedy, and the medicine has assisted his will. The same thing takes place in the inward man in relation to sin which is its lameness, by the grace of Him who |came not to call the righteous, but sinners;| since |the whole need not the physician, but only they that be sick.|