  The Phaedo was also known to ancient readers as Plato's On the Soul, whereas the Republic has On Justice as an alternative ancient title. Plato, however, conceives of justice as the excellent state of the soul, and so it is not surprising that the Republic sheds a great deal of light on Plato's conception of the soul. One way in which it does so is by explicitly integrating a number of central features of the ordinary notion of soul, features which, in the Phaedo, coexist somewhat uneasily: namely, responsibility for the life of an organism (that is, in the human case, responsibility for its being and remaining alive as a human being), for cognitive and (especially) intellectual functions, and for moral virtues such as courage and justice.  Towards the end of Republic 1, Socrates offers Thrasymachus an elaborate argument to the conclusion that “injustice is never more profitable than justice” (354a). If we set aside, as irrelevant to the dialectical context, the possibility that injustice and justice are equally profitable, it is clear that the conclusion here is equivalent to the position that the Republic is designed to establish, in response to Glaucon's request, at the beginning of Book 2, to be convinced by Socrates “that it is better in every way to be just than to be unjust” (357a). The argument at the end of Book 1 proceeds by attempting to prove an interim conclusion that is unnecessarily strong, namely that the just person is happy, whereas the unjust person is wretched. To establish the desired conclusion, it is enough to prove that the just person is always happier than the unjust person, which, unlike the unnecessarily strong interim conclusion, is compatible with the view that justice is not sufficient for (fully completed) happiness, since that requires suitable external circumstances in addition to justice. Nothing in Socrates' long answer to Glaucon (and Adeimantus) commits him to the view that justice is sufficient for (complete) happiness (cf. Irwin 1999). However, that view is not implied by the conception of the soul that Socrates relies on in this (Book 1) argument.  Moreover, nothing in the Republic contradicts or modifies this conception of the soul (on the contrary: cf.  445a9f., 609b f.), and so there is no reason not to take it seriously as a contribution to Plato's on-going reflection on the soul, even though the argument that surrounds it is designed to support a conclusion that Socrates subsequently succeeds in avoiding. 