In the preceding chapter where we took up fornication, we discussed also resorting to a courtesan, and by that we meant the arranged liaison of an unmarried man with a woman. In contrast, by taking a mistress here we mean the similarly arranged liaison of a married man with a woman. People who do not distinguish between classes of things use these two expressions indiscriminately, as though they had the same meaning and so the same implication. But since the circumstances are of two kinds, and the expression, resorting to a courtesan, fits the first, because a courtesan is a fallen woman, and the expression, taking a mistress, fits the second, because a mistress is a substitute partner of the bed, therefore, for the sake of distinction, we designate an arranged liaison with a woman prior to marriage by the expression, resorting to a courtesan, and an arranged liaison subsequent to marriage by taking a mistress. [2] We discuss the taking of a mistress here for the sake of an ordered consideration; for from an ordered consideration the nature of marriage is discovered on the one hand, and the nature of adultery on the other. We showed to begin with that marriage and adultery are opposed, in the chapter on the opposition of the two.* But the extent to which they are opposed, and in what way, cannot be assimilated except in the light of intermediate courses which lie in between, which include also the taking of a mistress. However, because this relationship is of two kinds, and kinds which must be altogether differentiated, therefore the present consideration, like previous ones, must be divided into its component parts. This we will do as follows:

(1) The taking of a mistress is of two kinds, which differ greatly from each other, one being in conjunction with the wife, the other in separation from the wife. (2) Taking a mistress in conjunction with the wife is altogether forbidden to Christians and abhorrent. (3) It is polygamy, which has been banned from the Christian world, and ought to be banned. (4) It is licentiousness, by which the conjugial inclination, the precious treasure of Christian life, is lost. (5) Taking a mistress in separation from the wife, when done for legitimate, just and truly weighty reasons, is not forbidden. (6) Legitimate reasons for taking a mistress in such a circumstance are the same as those for legitimate divorce, when the wife is nevertheless retained in the home. (7) Just reasons for taking a mistress in such a circumstance are the same as those for separation from the bed. (8) Weighty reasons for taking a mistress in such a circumstance are real and unreal. (9) Weighty reasons that are real are ones which are founded on justice. (10) But weighty reasons that are not real are ones which are not founded on justice, even though on an appearance of justice. (11) Men who take a mistress for legitimate, just and real weighty reasons may be at the same time in a state of conjugial love. (12) As long as this relationship with a mistress continues, physical conjunction with the wife is prohibited.

Explanation of these statements now follows. * See "The Opposition of Licentious Love to Conjugial Love," nos. 423ff.