α) The infirmity of human nature flows from four separate and distinct sources: (1) concupiscence (_fomes peccati_); (2) imperfection of the ethical judgment (_imperfectio iudicii_); (3) inconstancy of the will (_inconstantia voluntatis_); and (4) the weariness caused by continued resistance to temptation. In view of these agencies and their combined attack upon the will, theologians speak of a _necessitas antecedens peccandi_;—not as if the will were predestined to succumb to any one temptation in particular, but in the sense that it is morally unable to resist the whole series (_suppositione disiunctâ_). The will simply grows weaker and weaker, and in course of time fails to resist sin with sufficient energy. Let us exemplify. The proofsheets of a book are scrutinized by several trained readers, yet in spite of the greatest care and many ingenious devices for the elimination of error, a perfect book, _i.e._ one entirely free from mistakes, is a practical impossibility. How much harder must it be for man to avoid moral lapses throughout his whole life, considering that he cannot choose his own time for meeting temptations, but must keep his mind and will under constant control and be prepared to resist the enemy at any moment.(376) St. Thomas Aquinas says: “Man cannot avoid all venial sin, because his sensual appetite is depraved. True, reason is able to suppress the individual stirrings of this appetite. In fact, it is on this account that they are voluntary and partake of the nature of sin. But reason is not able to suppress them all [collectively], because, while it tries to resist one, there perhaps arises another, and, furthermore, reason is not always in a condition to exercise the vigilance necessary to avoid such impulses.”(377) It follows that the _necessitas peccandi antecedens_ does not destroy the liberty of the will or the moral imputability of those venial sins which a man actually commits; for it is merely a _necessitas indeterminata_, which refers not to certain particular instances, but to the one or other indeterminately. It follows further that God does not command the impossible when He insists that we should avoid venial sin, for He does not in each single case command something which is physically or morally impossible,(378) but merely demands a perfection which in itself is not entirely unattainable _hic et nunc_ with the assistance of ordinary grace.(379)