There are several different places in Plato's
texts in which it is stated or implied that virtue is sufficient for happiness.
First, it is helpful to point out that the good is often identified with happiness.
This is important because given this identity, then when one can be shown to be good, then
it follows that he is happy.
Thus, if virtue makes one good, and goodness is identical to happiness, then virtue makes
one happy.
This identification is most explicit in the Symposium when Diotima is questioning Socrates
and they both agree that to possess what is good is to be happy.
In the Gorgias, Socrates argues that the self-controlled person will be both altogether good (and here
he briefly restates the unity of the virtues) and happy.
The undisciplined and thus unvirtuous person will be licentious and unhappy.
In the Republic, this comes up several times.
Here the ultimate aim is the possession of the good or happiness (eudaimonia).
Again, remember that the two are identified.
This consists in doing good, which as such is accompanied by a feeling of pleasure.
That is the state of harmony in the soul, an inner intellectual and moral order.
It is the beauty and the health of the soul, the supremacy of the rational part in man
over the lower impulses.
The whole moral constitution of a man when it is as it should be is called justice.
He draws a picture of the ideal just man who does not doubt the rightness of his principles
even if he has to suffer the death of a criminal while the malefactor for his part triumphs.
He argues in the Republic that the just man will always be happier than the unjust man.
Furthermore, although justice is the kind of thing that is both valuable for its own
sake as well as for external rewards it brings, it is still the best thing for a man even
if he gets nothing out of it and suffers what the world calls a terrible fate.
Why?
Because mental and moral health depend on the right relationship or harmony between
the various elements in the soul, so that the highest of human desires, that for philosophy,
prevails, and the emotions and passions are gratified to an extent compatible with the
welfare of the whole.
It is not a repression of desire but a rechanneling of its course.
Right and wrong, justice and injustice, are not simply matters of the relationship between
man and man; essentially they are inward spiritual states of the individual, respectively a healthy
and a pathological state of the psyche.
The tyrannical man is enslaved and unhappy.
He is subject to all his desires and fears that can never be appeased.
He is at the mercy of the worst part of himself.
He is a slave to his basest passions.
Thus he is least able to do what he wishes, he is poor because he is never satisfied,
and he lives in constant fear.
At Republic 587e, Socrates claims that the philosopher king is 729 times happier than
the tyrant, though his math is in error: correcting his calculations, we find the philosopher
is only 125 times happier.