Summa summae S. Thomae, Tomus II

The question is of the perfect use of reason, which suffices for deliberating about things most weighty, to which a child arrives gradually and successively, some more quickly, some more slowly.

A conversion to God is distinguished two ways: one explicit and formal toward God distinctly and explicitly known, whether through faith, or through natural light; the other virtual and implicit, whereby one proposes to live according to reason, or loves the honest good, in which God, as author and end of this good, implicitly is contained. The instant in which the child who has reached the perfect use of reason deliberates about the last end, setting it before himself, is not physical but moral, comprising several physical instants; because however it takes place in the briefest time, it is called an instant.

I say: In the first moral instant of perfect use of reason man is bound under mortal sin to convert himself to God, either explicitly, in that manner in which he can and knows him, or, as the holy Doctor says, Hebr., V, 14, he who has his senses exercised to discern the good and evil that is of others ¹.

It is proved 1°. That which first occurs to man upon arriving at the perfect use of reason is to think and deliberate about himself, namely toward what end he is about to order himself and his actions. Therefore he is bound then to constitute God as his end, if he know him explicitly, or the good suitable to his nature, namely the honest good consonant with reason, in which God implicitly is contained. — Proof of the antecedent: Man is the end of all desirable things, but man himself is means with respect to God, who is his [man's] end. But the end is prior in intention. Therefore what first occurs to man arriving at the perfect use of reason is to think and deliberate about himself, toward what end he should order himself and his actions.

It is confirmed. To arrive at the perfect use of reason is to discern between moral good and evil; and thus at that instant the natural law, through synderesis dictating that the honest good is to be done and evil shunned, is intimated to him. Therefore man is bound then to accept that law and subject himself to it, proposing to act according to it; just as, while the evangelical law is sufficiently promulgated to anyone, he is bound straightway to accept it and subject himself to it, proposing to live and act according to it. Therefore.

It is proved 2°. There is a natural precept of converting oneself to God and loving him above all things, which, since it is affirmative, binds not always but for a determinate time. But there is no time more fitting, and in which it binds the more, than the first moral instant of use of reason. Therefore. — Proof of the minor in many ways: 1° From the reason already made, that what first occurs to man in this instant is to deliberate about himself in order to the end. 2° When man first arrives at the use of reason, he enters the world as a moral agent, and begins the way of the life of reason. Therefore then above all is he bound to set before himself the legitimate end, namely God, or the honest good, unto which direct all his actions: otherwise he is exposed forthwith to danger of going astray and losing salvation; as he who prudently begins a great journey ought to set before himself the term toward which he tends. 3° Because the other times assigned by theologians for the fulfillment of this precept are not so fitting.

I say 2°: It is probable that venial sin cannot be in anyone together with original alone ²

It is proved. If venial could be found in man together with original alone, this would happen either before the first instant of reason, or in that very instant, or immediately after. Not before the first instant of reason, because then man lacks the use of reason and liberty, and thus is incapable of venial sin; not in the very instant of the use of reason, because either he converts himself to God, and is justified from original sin, or converts himself not, and thus sins mortally, as has been said; consequently neither immediately after the first instant can there be venial together with original alone.