[http://www.newadvent.org/cathen/12783a.htm]
[The Catholic Encyclopedia]
...
Let it be taken for granted that workman and employer should, as a rule, make free agreements, and in particular should agree freely as to wages; nevertheless, there is a dictate of natural justice more imperious and ancient than any bargain between man and man, that remuneration should be sufficient to maintain the wage-earner in reasonable and frugal comfort.
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