Here’s proof of the ultimate limitation of science. The will can be moved by physics (if you like) but only by violence.
1 Now, we must not think that the souls of the heavens, if there be such, or any other created, separate, intellectual substances can directly insert a will-act into us or cause our act of choice to occur.
2 For the actions of all creatures are embraced under the order of divine providence, so they cannot operate outside its laws. But it is the law of providence that everything be moved immediately by its proximate cause. So, unless such an order were obeyed, a superior created cause could neither move nor do anything. Now, the proximate mover of the will is the good as apprehended, which is its object, and it is moved by it, just as sight is by color. So, no created substance can move the will except by means of a good which is understood. Now, this is done by showing it that something is a good thing to do: this is the act of persuading. Therefore, no created substance can act on the will, or be the cause of our act of choice, except in the way of a persuading agent.
Notes Hence both propaganda and education.
3 Again, a thing is by nature capable of being moved by, and of undergoing a passion from, an agent with a form by which the thing can be reduced to act, for every agent acts through its form. But the will is reduced to act by the desirable object which gives rest to its desire. Now, the will’s desire finds rest in the divine good only, as in its ultimate end, as is evident from what we said above. Therefore, God alone can move the will in the fashion of an agent.
4 Besides, as natural inclination in an inanimate thing, which is also called natural appetite, is related to its proper end, so also is the will, which is also called intellectual appetite, in an intellectual substance. Now, to give natural inclinations is the sole prerogative of Him Who has established the nature. So also, to incline the will to anything, is the sole prerogative of Him Who is the cause of the intellectual nature. Now, this is proper to God alone, as is evident from our earlier explanations. Therefore, He alone can incline our will to something.
Notes There is so much practical evidence of this that we didn’t need this technical proof. But it also means that the demons are under God’s control. So that if, perchance, they are pestering you, it is only by God’s permission or command. This is a most sobering thought.
5 Moreover, the violent, as is said in Ethics III , is “that whose principle is outside; the patient making no contribution of force.” So, if the will is moved by some external principle, the motion will be violent. Now, I am talking about being moved by some external principle which moves in the way of an agent, and not in the way of an end. But the violent is incompatible with the voluntary. So, it is impossible for the will to be moved by an extrinsic principle as by an agent; rather, every movement of the will must proceed from within. Now, no created substance is joined to the intellectual soul in regard to its inner parts, but only God, Who is alone the cause of its being and Who sustains it in being. Therefore, by God alone can voluntary movement be caused.
6 Furthermore, violent movement is opposed to natural and voluntary movement, because both of the latter must arise from an intrinsic source. The only way in which an external agent moves a thing naturally is by causing an intrinsic principle of motion within the movable thing. Thus, a generating agent, which gives the form of weight to a heavy generated body, moves it downward in a natural way.
No other extrinsic being can move a natural body without violence, except perhaps accidentally, by removing an impediment, and this uses a natural motion, or action, rather than causes it. So, the only agent that can cause a movement of the will, without violence, is that which causes an intrinsic principle of this movement, and such a principle is the very power of the will. Now, this agent is God, Who alone creates a soul, as we showed in Book Two. Therefore, God alone can move the will in the fashion of an agent, without violence.
7 Hence it is said in Proverbs (21:1): “The heart of the king is in the hand of the Lord; wherever He wishes, He turns it.” And again in Philippians (2:13): “It is God Who works in us, both to will and to accomplish, according to His good will.”