For MacIntyre those who inherited the Homeric canon were left with a dilemma regarding what constituted justice. Was it excellence, a justice founded upon a certain teleology toward which each person in their given station was supposed to work? Or was justice founded upon effectiveness, what works best for me? A justice founded on excellence exists prior to the making of law, in fact, laws are supposed to be created to serve that justice. With a justice founded on effectiveness, the meaning of justice is constituted by the act of legislating itself. The making of law and justice go hand in hand.
These two conceptions of justice produce two differing conceptions of practical rationality. The answer “what should I do?” depends greatly on how you understand justice. Within a framework that priorities excellence a good reason can be given without reference to the costs or benefits to the individual asking the question. You should do the right thing, irrespective of whether or not it will help or harm you. However, if you prioritize efficacy, then your answer must provide some compelling benefit or cost related to the action. What should I do? Whatever will bring me the most pleasure or help me to avoid the most pain.
MacIntyre’s thesis is that these two competing understandings of justice and rationality are expressed in competing political systems which give priority to one over the other and it is to the polis as the corporate embodiment of these different conceptions of justice that he then turns.