His definition of faith is an act of the intellect (mind) turned on by the will. But what is his definition of 'Will'? Is the will also an act of the mind or just moren an instinct/desire?
There are different understandings of Aquinas' position on the will. There is a dispute between voluntaristic and intellectualist interpretations as well as disagreement over developmental changes in Aquinas's view of the will. ( P. S. Eardley,'Thomas Aquinas and Giles of Rome on the Will', The Review of Metaphysics, Vol. 56, No. 4 (Jun., 2003), pp. 835-862.)
Basic account of the will
This account derives from the Summa Theologiae :
In Article 1 he distinguishes human actions properly so-called (actus humani) from actions merely associated with a human being (actus hominis) such as reflex movements, inadvertent beard scratching, nervous twitches, and the like. Human actions properly so-called are all and only those actions done by a human being insofar as he is a human being. A human being is a human being in virtue of possessing a rational soul, and so any action done by a human being that arises from his soul insofar as it is rational is an action done by him insofar as he is a human being. Aquinas thinks that possession of a rational soul endows one with certain cognitive powers - specifically intellectual powers - and with appetitive powers consequent on these intellectual powers.' He sometimes lumps the intellectual powers together under the heading "reason," and the appetitive powers specific to a rational creature, the rational appetite, he calls "will."' So human actions properly so-called are all and only those actions done by a human being that result from intellect (or reason) and will. Acting from intellect and will is the activity characteristic of human beings qua human beings.
What is it for an action to result from intellect and will? Aquinas devotes a great deal of attention to answering this question and the argument I am examining is part of that answer, so the barest sketch of the relevant features of his account will have to suffice for now. Aquinas thinks that the will is a natural inclination toward the good as it is conceived by intellect. Thus, for a human being to will to act it is necessary that the will be presented with an object conceived by the intellect as good. The process whereby intellect judges that some object or course of action is good is deliberation, and Aquinas calls the willing that follows on a deliberated judgment "deliberated willing."' To act from intellect and will, then, is to act from a volition arising from deliberation; and hence, human actions properly so-called are actions resulting from deliberated willing. (Since deliberation is properly an activity of the intellectual power and willing is properly an activity of the appetitive power, a deliberated willing is a rational desire.) (Scott MacDonald, 'Ultimate Ends in Practical Reasoning: Aquinas's Aristotelian Moral Psychology and Anscombe's Fallacy', The Philosophical Review, Vol. 100, No. 1 (Jan., 1991), pp. 31-66 : 35-7.) [Bold introduced by GT.]
Brief, accurate and clear answers on Aquinas, or indeed most philosophers, is difficult. I have explained Aquinas' concept of the will as well as I can. But see references.
Scott MacDonald, 'Ultimate Ends in Practical Reasoning: Aquinas's Aristotelian Moral Psychology and Anscombe's Fallacy', The Philosophical Review, Vol. 100, No. 1 (Jan., 1991), pp. 31-66.
P. S. Eardley, 'Thomas Aquinas and Giles of Rome on the Will', The Review of Metaphysics, Vol. 56, No. 4 (Jun., 2003), pp. 835-862.
Bonnie Kent, 'Aquinas and Weakness of Will', Philosophy and Phenomenological Research, Vol. 75, No. 1 (Jul., 2007), pp. 70-91.