Plotinos: Complete Works, v. 1 In Chronological Order,...

4. What then is this dialectics, knowledge of which must be added to mathematics? It is a science which makes us capable of reasoning about each thing, to say what it is, in what it differs from the others, in what it resembles them, where it is, whether it be one of the beings, to determine how many veritable beings there are, and which are the objects that contain nonentity instead of veritable essence. This science treats also of good and evil; of everything that is subordinated to (being), the Good, and to its contrary; of the nature of what is eternal, and transitory. It treats of each matter scientifically, and not according to mere opinion. Instead of wandering around the sense-world, it establishes itself in the intelligible world; it concentrates its whole attention on this world, and after having saved our soul from deceit, dialectics "pastures our soul in the meadow of truth,"347 (as thought Plato). Then it makes use of the Platonic method of division to discern ideas, to define each object, to rise to the several kinds of essences348 (as thought Plato); then, by thought concatenating all that is thence derived, dialectics continues its deductions until it has gone through the whole domain of the intelligible. Then, by reversing, dialectics returns to the very Principle from which first it had started out.349 Resting there, because it is only in the intelligible world that it can find rest, no longer needing to busy itself with a multitude of objects, because it has arrived at unity, dialectics considers its logic, which treats of propositions and arguments.

This logic is an art subordinate to dialectics just as writing is subordinate to thought. In logic, dialectics recognizes some principles as necessary, and others as constituting preparatory exercises. Then, along with everything else, subjecting these principles to its criticism, it declares some of them useful, and others superfluous, or merely technical.