I answer that, Since defense of the poor man's suit belongs to the works of mercy, the answer to this inquiry is the same as the one given above with regard to the other works of mercy (Question [32], Articles [5],9). Now no man is sufficient to bestow a work of mercy on all those who need it. Wherefore, as Augustine says (De Doctr. Christ. i, 28), "since one cannot do good to all, we ought to consider those chiefly who by reason of place, time, or any other circumstance, by a kind of chance are more closely united to us." He says "by reason of place," because one is not bound to search throughout the world for the needy that one may succor them; and it suffices to do works of mercy to those one meets with. Hence it is written (Ex. 23:4): "If thou meet thy enemy's ass going astray, bring it back to him." He says also "by reason of time," because one is not bound to provide for the future needs of others, and it suffices to succor present needs. Hence it is written (1 Jn. 3:17): "He that . . . shall see his brother in need, and shall put up his bowels from him, how doth the charity of God abide in him?" Lastly he says, "or any other circumstance," because one ought to show kindness to those especially who are by any tie whatever united to us, according to 1 Tim. 5:8, "If any man have not care of his own, and especially of those of his house, he hath denied the faith and is worse than an infidel."
It may happen however that these circumstances concur, and then we have to consider whether this particular man stands in such a need that it is not easy to see how he can be succored otherwise, and then one is bound to bestow the work of mercy on him. If, however, it is easy to see how he can be otherwise succored, either by himself, or by some other person still more closely united to him, or in a better position to help him, one is not bound so strictly to help the one in need that it would be a sin not to do so: although it would be praiseworthy to do so where one is not bound to. Therefore an advocate is not always bound to defend the suits of the poor, but only when the aforesaid circumstances concur, else he would have to put aside all other business, and occupy himself entirely in defending the suits of poor people. The same applies to a physician with regard to attendance on the sick.