To put the matter in another way, the equality which justice requires is wanting in an oppressive labour contract because of the inequality existing between the contracting parties. In the words of Professor Ely: "Free contract supposes equals behind the contract in order that it may produce equality."[216] Again, the rule of free contract is unjust because it takes no account of the moral claims of needs. A man whose only source of livelihood is his labour does wrong if he accepts a starvation wage willingly. Such a contract, however free, is not according to justice because it disregards the requirements of reasonable life. No man has a right to do this, any more than he has a right to perpetrate self mutilation or suicide. _The Rule of Market Value_ A third method of interpreting exchange equivalence is based upon the concept of value. Labour and compensation are thought to be equal when the value of one is equal to the value of the other. Then the contract is just and the compensation is just. The only objection to these propositions is that they are mere truisms. What does value mean, and how is it to be determined? If it is to receive an ethical signification; if the value of labour is to be understood as denoting not merely the value that labour will command in a market, but the value that labour ought to have,--the statement that wages should equal the value of labour becomes merely an identical proposition. All that it tells us is that wages ought to be what they ought to be. In its simplest economic sense value denotes purchasing power, or importance in exchange. As such, it may be either individual or social; that is, it may mean the exchange importance attributed to a commodity by an individual, or that attributed by a social group. In a competitive society social value is formed through the higgling of the market, and is expressed in market price. Now individual value is utterly impracticable as a measure of exchange equivalence in the wage contract. Since the value attributed to labour by the employer differs in the great majority of instances from that estimated by the labourer himself, it is impossible to determine which is the true value, and the proper measure of just wages.