16 Now after this dispensation righteous judgment will come: of which the Psalmist so speaks, as that we may understand that each man's punishment is wrought out of his own sin, and his iniquity turned into vengeance: that we may not suppose that that tranquillity and ineffable light of God brings forth from Itself the means of punishing sin; but that it so orders sins, that what have been delights to man in sinning, should be instruments to the Lord avenging. “Behold,” he says, “he has travailed with injustice.”
Now what had he conceived, that he should travail with injustice? “He has conceived,” he says, “toil.” Hence then comes that, “In toil shall you eat your bread.” Hence too that, “Come unto Me all you that toil and are heavy laden; for My yoke is easy, and My burden light.” For toil will never cease, except one love that which cannot be taken away against his will. For when those things are loved which we can lose against our will, we must needs toil for them most miserably; and to obtain them, amid the straitnesses of earthly cares, while each desires to snatch them for himself, and to be beforehand with another, or to wrest it from him, must scheme injustice.
Duly then, and quite in order, has he travailed with injustice, who has conceived toil. Now he brings forth what, save that with which he has travailed, although he has not travailed with that which he conceived? For that is not born, which is not conceived; but seed is conceived, that which is formed from the seed is born. Toil is then the seed of iniquity, but sin the conception of toil, that is, that first sin, to “depart from God.” He then has travailed with injustice, who has conceived toil. “And he has brought forth iniquity.” “Iniquity” is the same as “injustice:” he has brought forth then that with which he travailed. What follows next?
Source: The Enarrations, or Expositions, on the Psalms (New Advent)