 Liberalism is generally understood to arise historically out of the social contract tradition of political philosophy and hence rests on the idea of popular sovereignty. The concept of autonomy, then, figures centrally in at least one dominant strand in this tradition, the strand the runs through the work of Kant. The major alternative version of the liberal tradition sees popular sovereignty as basically a collective expression of rational choice and that the principles of the basic institutions of political power are merely instrumental in the maximization of aggregate citizen welfare (or, as with Mill, a constitutive element of welfare broadly considered). 