Summa contra Gentiles3.88 Chapter LXXXVIII. That created substances cannot be the direct causes of our acts of choosing and willing, but God alone

We must not think, however, that the souls of heavenly bodies, if there be any, or any intellectual separate substances, can directly impel our will or cause our choice.

For the actions of all creatures are subordinate to divine providence; so that they are unable to act beside its laws. Now it is a law of providence that everything is moved immediately by its proximate cause. Consequently, unless this order be observed, the higher created cause can neither move nor act. Again, the proximate moving cause of the will is the good understood, which is its object, and it is moved thereby as sight by colour. Therefore no created substance can move the will except by means of the good understood: in so far, to wit, as it shows it that a particular thing is good to do: and this is to persuade. Therefore no created substance can act on the will, or cause our choice, except by way of persuasion.

Again. A thing is naturally moved by and passive to that agent by whose form it can be reduced to act: since every agent acts by its form. Now the will is made actual by the appetible object, which stills the movement of its desire. And the will’s desire is satisfied by the divine good alone as its last end, as we have proved above. Therefore God alone can move the will as an agent.

Besides. The natural inclination, which we call the natural appetite, of inanimate things for their proper end is like the will or intellectual appetite in intellectual substances. Now a natural inclination cannot be given except by the maker of nature. Therefore the will cannot be inclined to anything except by the cause of the intellectual nature. But this belongs to God alone, as we have proved above. Therefore He alone can incline our will to anything.

Moreover. As stated in 3 Ethic, i., a violent action is one in which the principle is external, and the one who suffers violence contributes nothing. Consequently if the will be moved by an external principle, its movement will be violent:—and I speak of being moved by an external principle that moves as an agent, and not as an end. Now the violent is opposed to the voluntary. Therefore it is impossible that the will be moved by an external principle as an agent, and every movement of the will must come from within. But no created substance is united to the intellectual soul in its inmost being except God alone, who alone is the cause and sustainer of its being. Therefore the movement of the will can be caused by none but God alone.

Further. Violent movement is contrary to natural and voluntary movement; because both of these must be from an internal principle. But an external agent does not cause a natural movement except in so far as it causes an internal 1 Ch. xxxvii., 1. 2 Bk. II., ch. lxxxvii.

principle of movement to be in the movable thing: thus the generator that gives the form of gravity to the generated heavy body, gives it a natural downward movement. And nothing else external can move a natural body without violence, except perhaps indirectly, as that which removes an obstacle, for such a thing makes use of natural movement or action rather than causes it. Therefore that agent alone can cause a movement of the will without violence, which causes the internal principle of that movement, namely the power itself of the will. And this is God, who alone creates the soul, as we proved in the Second Book. Therefore God alone can move the will, as an agent, without violence.

This is expressed in the words of Prov. xxi. i: The heart of the King is in the hand of the Lord, whithersoever He will He shall turn it: and Philip ii. 13: It is God who worketh in us both to will and to accomplish, according to His good will.