A thing is said to be natural in two ways. First, as resulting of necessity from the principles of nature; thus upward movement is natural to fire. In this way matrimony is not natural, nor are any of those things that come to pass at the intervention or motion of the free-will. Secondly, that is said to be natural to which natureinclines although it comes to pass through the intervention of the free-will; thus acts of virtue and the virtues themselves are called natural; and in this way matrimony is natural, because natural reason inclines thereto in two ways. First, in relation to the principal end of matrimony, namely the good of the offspring. For nature intends not only the begetting of offspring, but also its education and development until it reach the perfect state of man asman, and that is the state of virtue. Hence, according to the Philosopher (Ethic. viii, 11,12), we derive three things from our parents, namely "existence," "nourishment," and "education." Now a child cannot be brought up and instructed unless it have certain and definite parents, and this would not be the case unless there were a tie between the man and a definite woman and it is in this that matrimony consists. Secondly, in relation to the secondary end of matrimony, which is the mutual services which married persons render one another in household matters. For just as natural reason dictates that men should live together, since one is not self-sufficient in all things concerning life, for which reason man is described as being naturally inclined to political society, so too among those works that are necessary for human life some are becoming to men, others to women. Whereforenature inculcates that society of man and woman which consists in matrimony. These two reasons are given by the Philosopher (Ethic. viii, 11,12).
Saturday, June 19, 2010
From Summa Theologica: