The ultimate telos for an architectonic legislator intending to establish a good constitution and laws, the previous chapters have argued, is eudaimonia. As an activity of the soul in accord with the best virtue (NE I.7, 1098a17), eudaimonia nonetheless presupposes a political community with just laws that are directed to the common advantage. Aristotle accordingly characterises the common advantage (to koine[i] sumpheron) as the aim of law and associates it with the promotion of eudaimonia in the polis (NE V.1, 1129b15-19). The common advantage may thus be regarded as the proximate final cause of legislative activity. Aristotle’s account of the common advantage nonetheless seems to equivocate between its status as an instrumental good serving the ends of individuals and as an intrinsically desirable state of affairs. In the current chapter, I seek to resolve this tension by conceptualising the common advantage as both a motivational reason leading individuals to enter the polis and a normative reason – identifiable with the political good of justice – which should guide the enactment of law. Section 1 considers some interpretative preliminaries to a correct understanding of the common advantage. Section 2 then examines the motivational role of the common advantage as a reason for political participation and its relation to instrumental and aggregative accounts of the common good. In section 3, I turn to the common advantage’s status as a normative reason identifiable with political justice. Section 4 then argues for the reconcilability of motivational and normative aspects of the common advantage by reference to the medieval interpretation of the polis as a unity of order.