Mathematics, which as a student by nature he will take very easily, will be prescribed to train him to abstract thought and to faith in the unembodied; a moral being by native disposition, he must be led to make his virtue perfect; after the Mathematics he must be put through a course in Dialectic and made an adept in the science. Enneads: I. III. 3
But this science, this Dialectic essential to all the three classes alike, what, in sum, is it? It is the Method, or Discipline, that brings with it the power of pronouncing with final truth upon the nature and relation of things – what each is, how it differs from others, what common quality all have, to what Kind each belongs and in what rank each stands in its Kind and whether its Being is Real-Being, and how many Beings there are, and how many non-Beings to be distinguished from Beings.
Dialectic treats also of the Good and the not-Good, and of the particulars that fall under each, and of what is the Eternal and what the not Eternal – and of these, it must be understood, not by seeming-knowledge (“sense-knowledge”) but with authentic science. Enneads: I. III. 4
But whence does this science derive its own initial laws? The Intellectual-Principle furnishes standards, the most certain for any soul that is able to apply them. What else is necessary, Dialectic puts together for itself, combining and dividing, until it has reached perfect Intellection. “For,” we read, “it is the purest (perfection) of Intellection and Contemplative-Wisdom.” And, being the noblest method and science that exists it must needs deal with Authentic-Existence, The Highest there is: as Contemplative-Wisdom (or true-knowing) it deals with Being, as Intellection with what transcends Being.
Is Dialectic, then, the same as Philosophy? It is the precious part of Philosophy. We must not think of it as the mere tool of the metaphysician: Dialectic does not consist of bare theories and rules: it deals with verities; Existences are, as it were, Matter to it, or at least it proceeds methodically towards Existences, and possesses itself, at the one step, of the notions and of the realities. Enneads: I. III. 5