PART VI
THE SIXTH AND NINTH COMMANDMENTS
IT is usual to treat of these two Commandments together, for the Sixth, " Thou shalt not commit adultery," [1] in expressly forbidding the chief sin, implicitly forbids all other external sins against the laws of marriage, and the Ninth, " Thou shalt not covet thy neighbour's house, neither shalt thou desire his wife," [2] forbids internal sins of covetousness and lust. The general doctrine concerning internal sins was given in a former Book; the special doctrine about covetousness in so far as it is against justice is clear from what was said about avarice and what will be said later about justice; the doctrine about external sins of lust will be evident from what has to be said in this place.
CHAPTER I
THE NATURE OF IMPURITY
I. THE means devised by God for the preservation and increase of the human race is the union of the sexes. This union has for its primary object the procreation of children, who require for their proper education the long and assiduous care of both father and mother. Nature, then, as well as the law of God and of the Church, requires that children should only be begotten of parents joined in lawful and indissoluble wedlock. As nature has taken care that the individual should take the food and drink necessary for his personal support by giving him the spur of appetite for nourishment and pleasure in taking it, so the same great Mother has taken care of the race by joining venereal pleasure to the act of procreating children. This venereal pleasure is lawful when indulged in between married people and according to the laws of marriage. In all other cases it is unlawful, and is forbidden by the Sixth and Ninth Commandments.
Venereal pleasure must be distinguished from sensual and from venereo-sensual pleasure. Venereal pleasure has its seat
