Summa contra Gentiles2.53 Chapter LIII. That in created intellectual substances there is act and potentiality

From the foregoing it is evident that in created intellectual substances there is composition of act and potentiality.

For in whatever thing we find two, of which one is the complement of the other, the ratio of one of them to the other is as the ratio of potentiality to act: since nothing is completed save by its proper act. Now in the created intellectual substance we find two things, namely its substance and its being, which is not its very substance, as we have proved. Now this very being is the complement of the existing substance, since a thing is actual by the fact that it has being. It follows therefore that in each of the aforesaid substances there is composition of act and potentiality.

Moreover. That which is received by a thing from an agent, must be an act: since it belongs to an agent to make a thing actual. Now it was proved above that all other substances have being from the first agent: and it is through having being from another that the substances thus caused exist. Consequently being is in the substances caused as an act of theirs. But that in which there is act, is a potentiality: since act as such refers to potentiality. Therefore in every created substance there is potentiality and act.

Again. Whatsoever participates a thing is compared to the thing participated as potentiality to act: since by that which is participated the participator is made to be actually such. Now it was shown above that God alone is essentially being, and all other things participate being. Therefore every created substance is compared to its being as potentiality to act.

Further. The likeness of a thing to its efficient cause results from act: because the agent produces its like in so far as it is in act. Now the likeness of every created substance to God is by being itself, as shown above. Therefore being is compared to all created substances as their act. Whence it follows that in every created substance there is composition of act and potentiality.