[Footnote 28: Plato, Republ. ii. pp. 371 E-372 A. [Greek: Pou= ou)=n a)/n pote e)n au)tê=| (tê=| po/lei) ei)/ê ê(/ te dikaiosu/nê kai\ ê( a)diki/a? . . . E)gô\ ou)k e(nnoô=, ei) mê/ pou _e)n au)tô=n tou/tôn chrei/a| tini\ tê=| pro\s a)llê/lous_.]] Here Plato expressly declares the principle of reciprocity to be the fundamental cause which generates and sustains the communion called the city. No man suffices to himself: every man has wants which require supply from others: every man can contribute something to supply the wants of others. Justice or injustice have place, according as this reciprocal service is carried out in one manner or another. Each man labours to supply the wants of others as well as his own. This is the primitive, constant, indispensable, bond whereby society is brought and held together. Doubtless it is not the only bond, nor does Plato say that it is. There are other auxiliary social principles besides, of great value and importance: but they presuppose and are built upon the fundamental principle--reciprocity of need and service--which remains when we reduce society to its lowest terms; and which is not the less real as underlying groundwork, though it is seldom enunciated separately, but appears overlaid, disguised, and adorned, by numerous additions and refinements. Plato correctly announces the reciprocity of need and service as one indivisible, though complex fact, when looked at with reference to the social communion. Neither of the two parts of that fact, without the other part, would serve as adequate groundwork. Each man must act, not for himself alone, but for others also: he must keep in view the requirements of others, to a certain extent, as well as his own. In his purposes and scheme of life, the two must be steadily combined. [Side-note: Antithesis and correlation of obligation and right. Necessity of keeping the two ideas together, as the basis of any theory respecting society.] It is clear that Plato--in thus laying down the principle of reciprocity, or interchange of service, as the ground-work of the social union--recognises the antithesis, and at the same time the correlation, between obligation and right. The service which each man renders to supply the wants of others is in the nature of an onerous duty; the requital for which is furnished to him in the services rendered by others to supply his wants. It is payment against receipt, and is expressly so stated by Plato--which every man conforms to, "believing that he will be better off thereby". Taking the two together, every man is better off; but no man would be so by the payment alone; nor could any one continue paying out, if he received nothing in return. Justice consists in the proper carrying on of this interchange in its two correlative parts.[29]