from the encyclical "Rerum novarum":
44. To this kind of argument a fair-minded man will not easily or entirely assent; it is not complete, for there are important considerations which it leaves out of account altogether. To labor is to exert oneself for the sake of procuring what is necessary for the various purposes of life, and chief of all for self-preservation. "In the sweat of thy face thou shalt eat bread." Hence, a man's labor necessarily bears two notes or characters. First of all, it is personal, inasmuch as the force which acts is bound up with the personality and is the exclusive property of him who acts, and, further, was given to him for his advantage. Secondly, man's labor is necessary; for without the result of labor a man cannot live, and self-preservation is a law of nature, which it is wrong to disobey. Now, were we to consider labor merely in so far as it is personal, doubtless it would be within the workman's right to accept any rate of wages whatsoever; for in the same way as he is free to work or not, so is he free to accept a small wage or even none at all. But our conclusion must be very different if, together with the personal element in a man's work, we consider the fact that work is also necessary for him to live: these two aspects of his work are separable in thought, but not in reality. The preservation of life is the bounden duty of one and all, and to be wanting therein is a crime. It necessarily follows that each one has a natural right to procure what is required in order to live, and the poor can procure that in no other way than by what they can earn through their work.
45. Let the working man and the employer make free agreements, and in particular let them agree freely as to the wages; nevertheless, there underlies a dictate of natural justice more imperious and ancient than any bargain between man and man, namely, that wages ought not to be insufficient to support a frugal and well behaved wage-earner. If through necessity or fear of a worse evil the workman accept harder conditions because an employer or contractor will afford him no better, he is made the victim of force and injustice.