St. Thomas held an ax in his hand.
He saw an instrument that he could hand to someone else–the ax. And he saw an instrument that was “his very own”–his hand.
In Chapter 41, he proposes this distinction as a solution to the one-Person-with-two-natures problem.
Father Mark White Blog 
Finally, we consider the problem of how Christ can be a single person even when his components may be considered persons.