According to these theories, the determining factor of wage justice is to be found in the wage contract. The basic idea is the idea of equality, inasmuch as equality is the fundamental element in the concept of justice. The principle of justice requires that equality should be maintained between what is owed to a person and what is returned to him, between the kinds of treatment accorded to different persons in the same circumstances. Similarly it requires that equality should obtain between the things that are exchanged in onerous contracts. An onerous contract is one in which both parties undergo some privation, and neither intends to confer a gratuity upon the other. Each exchanger desires to obtain the full equivalent of the thing that he transfers. Since each is equal in personal dignity an intrinsic worth to the other, each has a strict right to this full equivalent. Owing to the essential moral equality of all men, no man has a right to make of another a mere instrument to his own interests through physical force or through an onerous contract. Men have equal rights not only to subsist upon the earth, but to receive benefits from the exchange of goods. _The Rule of Equal Gains_ The agreement between employer and employé is an onerous contract; hence it ought to be made in such terms that the things exchanged will be equal, that the remuneration will be equal to the labour. How can this equivalence be determined and ascertained? Not by a direct comparison of the two objects, work and pay, for their differences render them obviously incommensurable. Some third term, or standard, of comparison is required in which both objects can find expression. One such standard is individual net advantage. Inasmuch as the aim of the labour contract is reciprocal gain, it is natural to infer that the gains ought to be equal for the two parties. Net gain is ascertained by deducting in each case the utility transferred from the utility received; in other words, by deducting the privation from the gross return. The good received by the employer when diminished by or weighed against the amount that he pays in wages should be equal to the good received by the labourer when diminished by or weighed against the inconvenience that he undergoes through the expenditure of his time and energy. Hence the contract should bring to employer and employé equal amounts of net advantage or satisfaction.