Thesis XI. -- Neither the matter of the universe, nor the human soul, nor anything else except the one simple infinitely perfect God, can be self-existent. Therefore all things except God are contingent.
82. The first two parts of this proposition are contained in the proposition just established. If nothing in the world known to us is inherent in the Divine substance, then neither matter nor human souls can be inherent in that substance. But outside the Divine substance there can be no self-existent substance, because self-existence is, as we have seen (Th. VII.-IX.), restricted necessarily to one simple infinitely perfect substance. Therefore the matter of the universe and human souls can have only conditioned existence, and are contingent substances.
The same argument proves that nothing outside of God can be self-existent. For if you assume anything else but God to be self-existent, for instance, if you assume with the Manichaeans a supreme principle of evil, you thereby destroy the unity, simplicity, and infinite goodness of self-existence clearly demanded by reason.
We need only remark that by "things" we mean realities in some way complete in themselves, endowed with an internal principle of action; such realities, for instance, as men and every living being that leads its own distinct life. All other realities diverse from the Divine substance are either parts of contingent things or accidental determinations of the same. In this way the human body is a part of the human substance, and the hands and feet of a man are parts of his body, whereas his sensations, thoughts, and volitions are accidental determinations. Since matter is contingent, and since only material substances can consist of parts, it is evident that all parts of substances are contingent. That accidental determinations of whatever contingent substances must be contingent, is implied by the very term "accidental," and follows, moreover, from their natural dependence upon contingent substances.
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