 One rationale for treating marital obligations as such promises might be thought to be the voluntaristic account of obligation. On this view, all special obligations (as opposed to general duties) are the result of voluntary undertakings; promises are then the paradigm of special obligations (see entry on Special Obligations). Thus, whatever special obligations spouses have to one another must originate in voluntary agreement, best understood as promise. We will return to this below. A second rationale is the assumption that existing marriage practices are morally arbitrary, in the sense that there is no special moral reason for their structure. Further, there are diverse social understandings of marriage. If the choice between them is morally arbitrary, there is no moral reason for spouses to adopt one specific set of marital obligations; it is up to spouses to choose their terms. Thus, the contractual account depends upon the assumption that there is no decisive moral reason for a particular marital structure.