NATURAL LAW ARGUMENTS CONCERNING the political order characteristically appeal, at some point or other, to the common good of the political community. To take the clearest example: Aquinas, perhaps the paradigmatic natural law theorist, appeals to the common good in his accounts of the definition of law, of the need for political authority, of the moral requirement to adhere to the dictates issued by political authority, and of the form political authority should take. But while united on the point that arguments for normative political conclusions must take the common good as a principle, natural law theorists have not been united in their understanding of the nature of the common good. The differences among natural law views on the character of the common good are not trivial: they concern such deep issues as whether the common good should be understood as an intrinsic or an instrumental good, and whether the common good should be understood in relation to the good of individuals of that community or solely in relation to the good of the community as a whole. If one aims to develop a natural law account of the political order, then, one cannot remain neutral with respect to the various natural law understandings of the common good, for these various understandings are almost certain to yield differing conclusions on the source, functions, and limits of political authority.
We can, I think, agree on some general formal considerations concerning the nature of the common good without presupposing a particular account of what the common good is; these formal considerations constrain the possible candidate conceptions and will help to determine which of the candidate conceptions is ultimately the most plausible. Not surprisingly, these considerations concern two features of the common good: its goodness and its commonness.
The notion that the common good must be good is meant to stand in place of a couple of theses concerning the way that the concept of the common good functions in governing political deliberation and in underwriting allegiance to the outcomes of that deliberation. Ends advanced by citizens in political deliberation as appropriate aims of state action are in fact such only if they can be brought under the description "necessary or useful to promoting, protecting, or honoring the common good." Given that political action is appropriately regulated by the aim of the common good, it seems that if we are not to disconnect political deliberation from the grounds for adherence to the outcomes of that deliberation, we need to say that the common good is something to which agents have good reasons to adhere. That the common good is the end regulating political deliberation and the source of allegiance to the outcomes of correct political deliberation generates both an epistemological and a practical constraint on theories of the content of the common good. The epistemological constraint is that inasmuch as the common good serves as a starting point for political deliberation, it must be something to which we can have some sort of cognitive access. The practical constraint is that a conception of the common good should be something that the citizen has--I will put it as vaguely as possible here--very strong reasons to pursue and to promote. I will try to be loose about this, noting only that it would be an unfortunate theory of the content of the common good that did not support a view in which citizens should have allegiance to the common good thus understood.
The other desideratum that a conception of the common good must satisfy is that it must be common. Put roughly, the idea is that the common good is common only if it is an end that is shared by reasonable agents within the political community. One might wonder whether this desideratum can serve in any way to differentiate the merits of competing conceptions of the common good: surely any barely plausible conception of the common good will meet this constraint. …