Happiness is attainable only through Justice or Virtue.

Justice is declared to be happiness, first, in itself, and secondly, in its consequences.

Such is the importance attached to this maxim as a safeguard of Society, that, whether true or not, it is to be maintained by state authority.
3.

The Psychology of Pleasure and Pain is given at length in the Philebus.
IV .-- With regard to the scheme of Duty.

In Plato, we find the first statement of the four Cardinal Virtues.
As to the Substance of the Moral Code, the references above made to the Republic and the Laws will show in what points his views differed from modern Ethics.
Benevolence was not one of the Cardinal Virtues.
His notions even of Reciprocity were rendered hazy and indistinct by his theory of Justice as an end in itself.
The inducements, means, and stimulants to virtue, in addition to penal discipline, are training, persuasion, or hortatory discourse, dialectic cognition of the Ideas, and, above all, that ideal aspiration towards the Just, the Good, around which he gathered all that was fascinating in poetry, and all the associations of religion and divinity.