Summa Theologica
Part I: Theology

The first part of the Summa is summed up in the premise that God governs the world as the "universal first cause ". God sways the intellect; he gives the power to know and impresses the species intelligibiles on the mind, and he sways the will in that he holds the good before it as aim, creating the virtus volendi. "To will is nothing else than a certain inclination toward the object of the volition which is the universal good." God works all in all, but so that things also themselves exert their proper efficiency. Here the Areopagitic ideas of the graduated effects of created things play their part in St. Thomas's thought.

Part I treats of God, who is the " first cause, himself uncaused" (primum movens immobile) and as such existent only in act (actu)—i.e. pure actuality without potentiality, and therefore without corporeality. His essence is actus purus et perfectus. This follows from the fivefold proof for the existence of God; namely, there must be a first mover, unmoved, a first cause in the chain of causes, an absolutely necessary being, an absolutely perfect being, and a rational designer. In this connection the thoughts of the unity, infinity, unchangeability, and goodness of the highest being are deduced.

As God rules in the world, the "plan of the order of things" preexists in him; in other words, his providence and the exercise of it in his government are what condition as cause everything which comes to pass in the world. Hence follows predestination: from eternity some are destined to eternal life, while as concerns others "he permits some to fall short of that end". Reprobation is more than mere foreknowledge; it is the "will of permitting anyone to fall into sin and incur the penalty of condemnation for sin".

The effect of predestination is grace. Since God is the first cause of everything, he is the cause of even the free acts of men through predestination. Determinism is deeply grounded in the system of St. Thomas; things (with their source of becoming in God) are ordered from eternity as means for the realization of his end in himself.

On moral grounds, St. Thomas advocates freedom energetically; but, with his premises, he can have in mind only the psychological form of self-motivation. Nothing in the world is accidental or free, although it may appear so in reference to the proximate cause. From this point of view, miracles become necessary in themselves and are to be considered merely as inexplicable to man. From the point of view of the first cause, all is unchangeable, although from the limited point of view of the secondary cause, miracles may be spoken of.

In his doctrine of the Trinity, Aquinas starts from the Augustinian system. Since God has only the functions of thinking and willing, only two processiones can be asserted from the Father; but these establish definite relations of the persons of the Trinity, one to another. The relations must be conceived as real and not as merely ideal; for, as with creatures relations arise through certain accidents, since in God there is no accident but all is substance, it follows that "the relation really existing in God is the same as the essence according to the thing". From another side, the relations as real must be really distinguished one from another. Therefore, three persons are to be affirmed in God.

Man stands opposite to God; he consists of soul and body. The "intellectual soul" consists of intellect and will. Furthermore, the soul is the absolutely indivisible form of man; it is immaterial substance, but not one and the same in all men (as the Averroists assumed). The soul's power of knowing has two sides: a passive (the intellectus possibilis) and an active (the intellectus agens).

It is the capacity to form concepts and to abstract the mind's images (species) from the objects perceived by sense; but since what the intellect abstracts from individual things is universal, the mind knows the universal primarily and directly and knows the singular only indirectly by virtue of a certain reflexio (cf. Scholasticism). As certain principles are immanent in the mind for its speculative activity, so also a "special disposition of works"—or the synderesis (rudiment of conscience)—is inborn in the "practical reason", affording the idea of the moral law of nature so important in medieval ethics.
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