The Art of Being Right/prelim
BY the ancients, Logic and Dialectic were used as synonymous terms; although logizesthai, "to think over, to consider, to calculate," and dialegesthai, "to converse," are two very different things.
The name Dialectic was, as we are informed by Diogenes Laertius, first used by Plato; and in the Phaedrus, Sophist, Republic, bk. vii., and elsewhere, we find that by Dialectic he means the regular employment of the reason, and skill in the practice of it. Aristotle also uses the word in this sense; but, according to Laurentius Valla, he was the first to use Logic too in a similar way. Dialectic, therefore, seems to be an older word than Logic. Cicero and Quintilian use the words in the same general signification.
This use of the words as synonymous terms lasted through the Middle Ages into modern times; in fact, until the present day. But more recently, and in particular by Kant, Dialectic has often been employed in a bad sense, as meaning "the art of sophistical controversy"; and hence Logic has been preferred, as of the two the more innocent designation. Nevertheless, both originally meant the same thing; and in the last few years they have again been recognised as synonymous.
It is a pity that the words have thus been used from of old, and that I am not quite at liberty to distinguish their meanings. Otherwise, I should have preferred to define Logic (from logos, "word" and "reason," which are inseparable) as "the science of the laws of thought, that is, of the method of reason"; and Dialectic (from dialegesthai, "to converse" - and every conversation communicates either facts or opinions, that is to say, it is historical or deliberative) as "the art of disputation," in the modern sense of the word. It is clear, then, that Logic deals with a subject of a purely à priori character, separable in definition from experience, namely, the laws of thought, the process of reason or the logos; the laws, that is, which reason follows when it is left to itself and not hindered, as in the case of solitary thought on the part of a rational being who is in no way misled. Dialectic, on the other hand, would treat of the intercourse between two rational beings who, because they are rational, ought to think in common, but who, as soon as they cease to agree like two clocks keeping exactly the same time, create a disputation, or intellectual contest. Regarded as purely rational beings, the individuals would, I say, necessarily be in agreement, and their variation springs from the difference essential to individuality; in other words, it is drawn from experience.
Logic, therefore, as the science of thought, or the science of the process of pure reason, should be capable of being constructed à priori. Dialectic, for the most part, can be constructed only à posteriori; that is to say, we may learn its rules by an experiential knowledge of the disturbance which pure thought suffers through the difference of individuality manifested in the intercourse between two rational beings, and also by acquaintance with the means which disputants adopt in order to make good against one another their own individual thought, and to show that it is pure and objective. For human nature is such that if A. and B. are engaged in thinking in common, and are communicating their opinions to one another on any subject, so long as it is not a mere fact of history, and A. perceives that B.'s thoughts on one end the same subject are not the same as his own, he does not begin by revising his own process of thinking, so as to discover any mistake which he may have made, but he assumes that the mistake has occurred in B.'s. In other words, man is naturally obstinate; and this quality in him is attended with certain results, treated of in the branch of knowledge which I should like to call Dialectic, but which, in order to avoid misunderstanding, I shall call Controversial or Eristical Dialectic. Accordingly, it is the branch of knowledge which treats of the obstinacy natural to man.
Eristic is only a harsher name for the same thing.
Original footnotes 
- He speaks of duochereiai logikai, that is, "difficult points," protasis logike, aporia logike.
- Cic. in Lucullo: Dialecticam inventam esse, veri et falsi quasi disceptatricem. Topica, c. 2: Stoici enim judicandi vias diligenter persecuti sunt, ea scientia, quam Dialecticen appellant. Quint., lib. ii., 12: Itaque haec pars dialecticae, sive illam disputatricem dicere malimus; and with him this latter word appears to be the Latin equivalent for Dialectic. (So far according to "Petri Rami dialectics, Audomari Talaei praelectionibus illustrata". 1569.)