§ 6. We have next to consider the duties of Affection that arise out of relationships voluntarily assumed. Of these the most important is the Conjugal Relation. And here we may begin by asking whether it be the duty of human beings generally to enter into this relation. It is no doubt normal to do so, and most persons are prompted to it by strong desires: but in so far as it can be said to be prescribed by Common Sense, it does not seem an independent duty, but derivative from and subordinate to the general maxims of Prudence and Benevolence.[198] And in all modern civilised societies, law and custom leave the conjugal union perfectly optional: but the conditions under which it may be formed, and to a certain extent the mutual rights and duties arising out of it, are carefully laid down by law; and it is widely thought that this department of law more than others ought to be governed by independent moral principles, and to protect, as it were, by an outer barrier, the kind of relation which morality prescribes. If we ask what these principles are, Common Sense--in modern European communities--seems to answer that the marriage union ought to be (1) exclusively monogamic, (2) at least designed to be permanent, and (3) not within certain degrees of consanguinity. I do not, however, think that any of these propositions can on reflection be maintained to be self-evident. Even against incest we seem to have rather an intense sentiment than a clear intuition; and it is generally recognised that the prohibition of all but monogamic unions can only be rationally maintained on utilitarian grounds.[199] As regards the permanence of the marriage-contract all would no doubt agree that fidelity is admirable in all affections, and especially in so close and intimate a relation as the conjugal: but we cannot tell _a priori_ how far it is possible to prevent decay of love in all cases: and it is certainly not self-evident that the conjugal relation ought to be maintained when love has ceased; nor that if the parties have separated by mutual consent they ought to be prohibited from forming fresh unions. In so far as we are convinced of the rightness of this regulation, it is always, I think, from a consideration of the generally mischievous consequences that would ensue if it were relaxed.