254. XVIII. THE THIRD CAUSE OF LEGITIMATE SEPARATION IS IMPOTENCE BEFORE
MARRIAGE. The reason why this is a cause of separation is, because the
end of marriage is the procreation of children, which cannot take place
where this cause of separation operates; and as this is foreknown by the
parties, they are deliberately deprived of the hope of it, which hope
nevertheless nourishes and strengthens their conjugial love.

255. XIX. ADULTERY IS THE CAUSE OF DIVORCE. There are several reasons
for this, which are discernible in rational light, and yet at this day
they are concealed. From rational light it may be seen that marriages
are holy and adulteries profane; and thus that marriages and adulteries
are diametrically opposite to each other; and that when opposites act
upon each other, one destroys the other even to the last spark of its
life. This is the case with conjugial love, when a married person
commits adultery from a confirmed principle, and thus from a deliberate
purpose. With those who know anything of heaven and hell, these things
are more clearly discernible by the light of reason: for they know that
marriages are in and from heaven, and that adulteries are in and from
hell, and that these two cannot be conjoined, as heaven cannot be
conjoined with hell, and that instantly, if they are conjoined with man
(_homo_), heaven recedes, and hell enters. Hence then it is, that
adultery is the cause of divorce; wherefore the Lord saith, that
"_whosoever shall put away his wife, except for whoredom, and shall
marry another, committeth adultery_," Matt. xix. 9. He saith, if, except
for whoredom, he shall put away his wife, and marry another, he
committeth adultery; because putting away for this cause is a plenary
separation of minds, which is called divorce; whereas other kinds of
putting away, grounded in their particular causes are separations, of
which we have just treated; after these, if another wife is married,
adultery is committed; but not so after a divorce.