In adjudicating between two opposite errors of political (and thus anthropological) philosophy, Jacque Maritain capitalizes on the distinction between our material individuality and our personhood. These are not, as he points out, two separate realities, but two aspects of one reality.
By the fact of our materiality, we are individuals: needy, fragile, and insufficient of our own. By reason of our personhood we are ordered toward certain perfections (both natural and supernatural). The natural perfections we are ordered to and our material needs render us parts of a greater whole. We receive these perfections by forming part of a political community which must presuppose the family as well as the person as such.
Insofar as this is true, our freedom, our desires, our goals, are all in some way subordinated and directed to the common good of that community. Thomas uses the analogy of the extremities of the body which by instinct will protect the head if something should threaten it.1 The hand will sacrifice itself for the greater good. The hand does so because the life or good of the whole is the good of the hand or that from which its proper good flows. There is no hand without the head, except equivocally.
Unlike the hand, the person within a society is himself a proper whole. As a person he is ordered to a good which excels all created goods. Thus society or the political community does not have absolute authority or ordination over the individual. But this does not mean that the body, the very life of the individual may not be wagered or offered on behalf of the community. However, it can rightly only be offered in freedom and for a true common good. It can only be so offered because the common good is the good of persons and in some way (either immediately or mediately) is directed to a Supreme good. He who offers his life in such a manner does not cast it away but aims at and somehow participates in their own highest end. The do not become reduced to a mere part.
Maritain offers this analogy:
“A good runner engages the whole of himself in the race but not by reason of all the functions or all the finalities of his being.”1
The runner may be wholly engaged as an individual in the act of running, but the act does not summarize the entirety of who they are. This is why, for instance, a person who is running may at the same time run to save someone’s life. All that the city may demand does not exhaust what the person is, and yet may be done in such a manner that it ordered to the love of God and neighbor. But this means these two are not equal realities. Rather, one of these callings is primary and ultimate. It is only in preserving this distinction and order that the person lives into their dignity as persons. It is in aiding and preserving this extrinsic good that human city achieves its own proper perfection, a common good, which is very good, but infinitely less good than incommunicable blessedness of the Saints.
1 Aquinas, Thomas Summa Theologica I-I Q.60, A.5, from: http://www.newadvent.org/summa/1060.htm
2 Maritain, Jacques, The Person and the Common Good, from: https://maritain.nd.edu/jmc/etext/CG04.HTM