Two things are competent to man in respect to exterior things. One is the power to procure and dispense them and in this regard it is lawful for man to possess property. Moreover this is necessary to human life for three reasons. First, because every man is more careful to procure what is for himself alone than that which is common to many or to all: since each one would shirk the labor and leave to another that which concerns the community, as happens when there are a great number of servants. Secondly, because human affairs are conducted in more orderly fashion if each man is charged with taking care of some particular thing himself, whereas there would be confusion if everyone had to look after any one thing indeterminately. Thirdly, because a more peaceful state is ensured to man if each one is contented with his own. Hence is to be observed that quarrels arise more frequently where there is no division of things possessed. (Sum. Theo., II-III, q. 66, art. 2.)
It would probably be a mistake to read too much modern economic thought into this paragraph, but he does, in a short passage, summarize the tragedy of the commons (sentence 2), allude to specialization and division of labor (sentence 3), and hint at the Coase theorem (sentence 4).
Not bad for one paragraph.