John Hardon is your new pope?

No.

But you certainly aren't.

So does a cat, a dog, a guinea pig, a horse... why didn't you argue that they also have the use of reason?

I feel there is a difference between instinct and reason.  If you can prove everything a child who has not reached the age of reason, and a lunatic does are based solely on instinct, perhaps I could agree with you.

Quam Singulari

The point here is that the Decree on Justification is talking about people with the use of reason, people with understanding and rationality.  It is so blatantly obvious from the very context of the decree itself, that I wonder why we are having this debate.

...whereas all men had lost their innocence in the prevarication of Adam-having become unclean, and, as the apostle says, by nature children of wrath...

...under the power of the devil and of death...

...not even the Jews by the very letter itself of the law of Moses, were able to be liberated, or to arise, therefrom; although free will, attenuated as it was in its powers, and bent down, was by no means extinguished in them...

...By which words, a description of the Justification of the impious is indicated...

...And this translation, since the promulgation of the Gospel, cannot be effected, without the laver of regeneration, or the desire thereof, as it is written; unless a man be born again of water and the Holy Ghost, he cannot enter into the Kingdom of God...

...The Synod furthermore declares, that in adults, the beginning of the said Justification is to be derived from the prevenient grace of God...

...neither is man himself utterly without doing anything while he receives that inspiration, forasmuch as he is also able to reject it; yet is he not able, by his own free will, without the grace of God, to move himself unto justice in His sight...

...and when, understanding themselves to be sinners, they, by turning themselves, from the fear of divine justice whereby they are profitably agitated, to consider the mercy of God, are raised unto hope...

...For even as no pious person ought to doubt of the mercy of God, of the merit of Christ, and of the virtue and efficacy of the sacraments, even so each one, when he regards himself, and his own weakness and indisposition, may have fear and apprehension touching his own grace; seeing that no one can know with a certainty of faith, which cannot be subject to error, that he has obtained the grace of God...

...Having, therefore, been thus justified, and made the friends and domestics of God, advancing from virtue to virtue, they are renewed, as the Apostle says, day by day; that is, by mortifying the members of their own flesh, and by presenting them as instruments of justice unto sanctification...

...But no one, how much soever justified, ought to think himself exempt from the observance of the commandments...