 The main theoretical alternatives to the contractual view hold that marital obligations are defined by the purpose of the institution and that spouses cannot alter these institutional obligations (much like the professional moral obligations of a doctor; to become a doctor, one must voluntarily accept the role and its obligations, and one cannot negotiate the content of these obligations). The challenge for institutional views is to defend such a view of marriage, explaining why spouses may not jointly agree to alter obligations associated with marriage. Kant confronted this question, arguing that special marital rights were morally necessary for permissible sex. His account of sexual objectification has influenced a prominent contemporary rival to the contractual view—the new natural law view, which takes procreation as essential to marriage. A second widespread approach focuses solely on love as the defining purpose of marriage.