It seems somewhat surprising that the question should have been raised, even theoretically, whether the Mosaic law, as regards the degrees of affinity prohibited in marriage, is of permanent authority. The reasons for these prohibitions, wherever given, are as valid now as then; for the simple reason that they are grounded fundamentally in a matter of fact,--namely, the nature of the relation between husband and wife, whereby they become "one flesh," implied in such phraseology as we find in ver. 16; and also the relation of blood between members of the same family, as in vv. 10, etc. Happily, however, whatever theory any may have held, the Church in all ages has practically recognised every one of these prohibitions, as binding on all persons; and has rather been inclined to err, if at all, by extending, through inference and analogy, the prohibited degrees even beyond the Mosaic code. So much, however, by way of guarding against excess in such inferential extensions of the law, we must certainly say: according to the law itself, as further applied in chap. xxi. 1-4, and limited in Deut. xxv. 5-10, relationship by marriage is not to be regarded as precisely equivalent in degree of affinity to relationship by blood. We cannot, for instance, conceive that, under any circumstances, the prohibition of the marriage of brothers and sisters should have had any exception; and yet, as we have seen, the marriage between brother and sister-in-law is explicitly authorised, in the case of the levirate marriage, and by implication allowed in other cases, by the language of ver. 18 of this chapter.