I'm laughing (NOT AT you) because rarely does Aquinas get accused of imprecision!
Check it out: http://www.newadvent.org/summa/3.htm And then root around for will. NOT to persuade but just to see another guy working in his unique way through the questions.
Not only is will an imprecise and arbitrary term, Aquinas claims it’s driven by appetites. Appetites are of course emotions, feelings, ect... and not rational decision making processes based on rationally chosen values. These explanations are rooted in ignorance.