Mkataba wa kuajiri mfanyakazi unahusu manufaa anayoyatoa kupitia juhudi yake; na ujira hupimwa kwa kuzingatia manufaa haya. Juhudi yenyewe si kipimo cha ujira, wala si kipimo cha manufaa, kwani vinginevyo ujira wa mchonga mawe ungekuwa mkubwa kuliko ujira wa mhandisi kwa sababu juhudi ya mchonga mawe ni kubwa zaidi; na hili ni kinyume cha uhalisia. Kwa hiyo, ujira ni malipo ya manufaa, si malipo ya juhudi. Zaidi ya hayo, ujira hutofautiana na kubadilika kulingana na aina ya mfanyakazi, na pia hubadilika kwa mfanyakazi yule yule kulingana na tofauti katika kiwango cha manufaa, lakini si kulingana na tofauti za juhudi. Katika hali zote mbili, mkataba ulihusu manufaa ya mwajiri, si juhudi ya mfanyakazi. Kwa hiyo kinachojali ni matokeo, iwe ni ya wafanyakazi tofauti katika kazi tofauti, au ya wafanyakazi tofauti katika kazi hiyo hiyo; na hakuna uzingatiaji wowote wa juhudi kabisa. Ni kweli kwamba matokeo ya kazi ni matunda ya juhudi, iwe ni katika kazi tofauti, au katika kazi hiyo hiyo inayofanywa na watu tofauti, lakini kinachokusudiwa ni matokeo, si juhudi peke yake, ingawa hili hujulikana katika kupima ujira. Kwa hiyo, mtu akiajiriwa kwa ujenzi, basi ujira unapaswa kupimwa kwa muda au kwa kazi. Ikipimwa kwa kazi, manufaa yatajidhihirisha wazi katika mahali pa jengo, urefu wake, upana wake, unene wake, na nyenzo za jengo n.k. Ikipimwa kwa muda, basi manufaa ya kazi kwa kawaida huongezeka muda unapoongezeka, na hupungua muda unapopungua. Hivyo, maelezo ya kazi pamoja na kutajwa kwa muda ndicho kipimo cha manufaa. Ikipimwa kwa muda, mtu hapaswi kufanya kazi zaidi ya uwezo wake wa kawaida, wala hapaswi kulazimishwa kufanya kazi ngumu isiyo ya kawaida.

The contract to hire an employee applies on the benefit of the effort He expends; and the wage is evaluated in terms of this benefit. The effort itself is neither the measure of the wage, nor the measure of the benefit, otherwise the wage of the stonemason would be greater than the wage of the engineer because the stonemason’s effort is greater; and this is contrary to the reality. Therefore, the wage is a recompense for the benefit and not for the effort. Besides that, the wage differs and changes according to the type of employee, and it also changes for the same employee according to the difference in the standard of the benefit, but not according to differences in effort. The contract in both cases was over the benefit of the employer, not over the employee’s effort. So what does matter is the result, whether it was of different employees in different works, or of different employees in the same work; and there is no consideration given to the effort at all. It is true that the result of the work is the fruit of the effort, whether it was in different works, or in the same work done by different people, but what is intended is the result, not only the effort, even though this is noticed in the evaluation of the wage. So if a person was hired for building, then the wage should be evaluated by the time or by the work. If it was evaluated by the work, then the benefit will obviously be manifested in the location of the building, its length, width, thickness and the material of the building etc. If the work was evaluated by time, then the benefit of the work usually increases as the time increases, and decreases as the time decreases. Thus the description of the work together with the mentioning of the time is the measure of the benefit. If it is evaluated by time, the person should not work more than his usual capacity, and should not be obliged to do unusually hard labour.