Aquinas states in the Summa Theologiae, First Part of the Second Part, Question 1, Article 5 the following:
... just as in the process of reasoning, the principle is that which is naturally known, so in the process of the rational appetite, i.e. the will, the principle needs to be that which is naturally desired.
For Aquinas, the will must originate in a first cause that is not deliberately chosen, for the same reason that reason must originate in a first principle that is not demonstrated. Otherwise we have ourselves an infinite regress.
While this much is obvious, things become murky later on when Aquinas identifies this first principle of the will (ST 1, q. 60, a. 3):
... the angel and man naturally desire their own good and their own perfection. And this is to love oneself. Hence both the angel and man naturally love themselves insofar as each desires some good for himself by a natural appetite.
It remains unclear what Aquinas is specifically referring to as the first principle of the will. It seems that there are two options that Aquinas himself specifies elsewhere (ST I-II, q.26, a.4):
... the motion of love tends towards two things: namely, toward some good which one wills for someone, either for one's self or for another; and toward that for which one wills this good. Thus one loves the good that is willed for the other with love of concupiscence, and that for which the good is willed with a love of friendship.
The following simplified definitions are provided:
1.) Love of Friendship: Love of a thing for its own sake
2.) Love of Concupiscence: Love of a thing for the sake of another thing
So the question is, does Aquinas identify the sort of self-love he deems to be the first principle of all human action as a love of friendship or a love of concupiscence?
Put in another way, is the first principle of human action a love of ourselves or a love of that which is good for ourselves?
It seems to me that a paradox arises:
We only love that which is good for us if we love ourselves, but we only love ourselves if doing so is good for us to do.
How would Aquinas reply to all of this?