For that is an absurd position which is taken by some people, who say that they will not rob a parent or a brother for their own gain, but that their relation to the rest of their fellow-citizens is quite another thing. Such people contend in essence that they are bound to their fellow-citizens by no mutual obligations, social ties, or common interests. This attitude demolishes the whole structure of civil society.

[Nam illud quidem absurdum est, quod quidam dicunt, parenti se aut fratri nihil detracturos sui commodi causa, aliam rationem esse civium reliquorum. Hi sibi nihil iuris, nullam societatem communis utilitatis causa statuunt esse cum civibus, quae sententia omnem societatem distrahit civitatis.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher

De Officiis [On Duties; On Moral Duty; The Offices], Book 3, ch. 6 (3.6) / sec. 28 (44 BC) [tr. Miller (1913)]
    (Source)

(Source (Latin)). Alternate translation:

For as to what is usually said by some men, that they would not take anything away from a father or brother for their own advantage, but that there is not the same reason for their ordinary citizens, it is foolish and absurd: for they thrust themselves out from partaking of any privileges, and from joining in common with the rest of their citizens, for the public good; an opinion that strikes at the very root and foundation of all civil societies.
[tr. Cockman (1699)]

That indeed is absurd, which some men avow, that for their own advantage they would take nothing from a parent or a brother; but that the case of other citizens is different. These men, stablish with their fellow-citizens no common right, no society for common advantage; an opinion that unhinges the whole internal intercourse of a state.
[tr. McCartney (1798)]

For that which some say, that they would take nothing wrongfully, for the sake of their own advantage, from a parent or brother, but that the case is different with other citizens, is indeed absurd. These establish the principle that they have nothing in the way of right, no society with their fellow citizens, for the sake fo the common interest -- an option which tears asunder the whole social compact.
[tr. Edmonds (1865)]

For this is absurd indeed which some say, that they would take nothing from a parent or a brother for their own benefit, but that it is quite another thing with persons outside of one’s own family. These men disclaim all mutual right and partnership with their fellow-citizens for the common benefit, -- a state of feeling which dismembers the fellowship of the community.
[tr. Peabody (1883)]

It is absurd for people to say that they will not despoil a father or a brother for their own advantage but that fellow-citizens stand on quite a different footing. That is practically to assert that they are bound to their fellow-citizens neither by mutual obligations, social ties, nor common interests. But such a theory tears in pieces the whole fabric of civil society.
[tr. Gardiner (1899)]

The contention that some people advance is absurd, of course: they argue that they would not deprive a parent or brother of anything for their own advantage but that there is another standard applicable to all other citizens. These people do not submit themselves to any law or to any obligation to cooperate with fellow citizens for the common benefit. Their attitude destroys any cooperation within the city.
[tr. Edinger (1974)]