De Regno cap. 2 n. 14:
Now since man must live in a group, because he is not sufficient unto himself to procure the necessities of life were he to remain solitary, it follows that a society will be the more perfect the more it is sufficient unto itself to procure the necessities of life. There is, to some extent, sufficiency for life in one family of one household, namely, insofar as pertains to the natural acts of nourishment and the begetting of offspring and other things of this kind. Self-sufficiency exists, furthermore, in one street with regard to those things which belong to the trade of one guild. In a city, which is the perfect community, it exists with regard to all the necessities of life. Still more self-sufficiency is found in a province because of the need of fighting together and of mutual help against enemies.
I didn't understand what Thomas Aquinas meant by "necessities of life" when talking about the self-sufficiency of the city. And I also don't understand how the city can be self-sufficient in all "necessities of life" if it, unlike the province, is incapable of dealing with war issues on its own.
This passage from the Summa Theologica I-II q. 105 a. 4 co. perhaps explains what Thomas meant by "necessities of life", but I still cannot understand how the city can be self-sufficient if there are things it cannot do alone (unlike the province, for example):
I answer that, The mutual relations of the members of a household regard everyday actions directed to the necessities of life, as the Philosopher states (Polit. i, 1). Now the preservation of man's life may be considered from two points of view. First, from the point of view of the individual, i.e. in so far as man preserves his individuality: and for the purpose of the preservation of life, considered from this standpoint, man has at his service external goods, by means of which he provides himself with food and clothing and other such necessaries of life: in the handling of which he has need of servants. Secondly man's life is preserved from the point of view of the species, by means of generation, for which purpose man needs a wife, that she may bear him children.