You don't need a constitution for this purpose, even though a constitution can have practical benefit. And furthermore, you'd only be passing the buck by appealing to a constitution. I could just ask whether the constitution is arbitrary, and in this case, you could not appeal to anything to ground or constrain it. The ultimate ground for the written law is the moral law, such that the first is a determination of the second given the circumstances of a given jurisdiction. Note also the distinction between lex and ius.