Most liberal constitutional theorizing, as exemplified by Buchanan (1975) and Rawls (1971), treats societies as closed systems of human interaction. This treatment is carried forward by emphasizing the similarity between choosing constitutional rules and choosing the rules for playing a parlor game such as poker. The menu of possible rules and the desires and values of the players are taken as data, and the players agree on the rules by which they will play. We don’t deny the heuristic value of this two-stage scheme of analysis; however, we also think that exploring constitutional thought from within a framework of open system also offers useful analytical insight, as we set forth here. In our alternative framework, agreement on rules is always incomplete, for two sets of reasons. One is the limited and divided quality of knowledge (Hayek 1937, 1945). The other is the persistent present of antagonism within society, as conveyed by Carl Schmitt’s (1932) distinction between friends and enemies, and with that distinction present as well in William Riker’s (1962) theory of political coalitions.