 To argue successfully that human well-being is increasing over the long term,  theorists of progress must offer an interpretation of well-being compatible with that claim. They are committed either to interpret human well-being as a  single value, or as a set of incommensurable values that are empirically connected.  In the first case, value monism, the list of compelling alternatives is not long.  It includes freedom, happiness or utility, and the realization of human  capabilities.  In the second case, theorists can draw on a wider range of values,  but will have to show that the incommensurable components of human well-being reinforce each other causally or at least do not clash.  As we will see, faced with the difficulties of the task, some theorists do not define well-being rigorously.  They may, however, formulate a conception of improvement for a circumscribed domain of social life, the description of which is a part of their overall account. 