Virtues are divided into ethical and intellectual (dianoetic); and so are the contrary vices.
These intellectual virtues are called by Aristotle dianoetic.
Happiness, therefore, consists in the combination of dianoetic and ethical virtues.
But the aim is after all the life of the intellect, and the "dianoetic" virtues are superior to the practical.
The three heads signify the intellective, dianoetic, and doxatic powers.
On this double requirement he founded two kinds of virtue, the intellectual (dianoetic) and the practical.