By the ancients, Logic and Dialectic were used as synonymous terms; although [Greek: logizesthai], "to think over, to consider, to calculate," and [Greek: 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. 
[Footnote 1: He speaks of [Greek: dyscherelai logicai], that is, "difficult points," [Greek: protasis logicae aporia logicae]]
[Footnote 2: 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 dialectica, Audomari Talaei praelectionibus illustrata." 1569.)]
This use of the words and 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 [Greek: 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 [Greek: 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 it 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 [Greek: 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 and 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.
Controversial Dialectic is the art of disputing, and of disputing in such a way as to hold one's own, whether one is in the right or the wrong—per fas et nefas.  A man may be objectively in the right, and nevertheless in the eyes of bystanders, and sometimes in his own, he may come off worst. For example, I may advance a proof of some assertion, and my adversary may refute the proof, and thus appear to have refuted the assertion, for which there may, nevertheless, be other proofs. In this case, of course, my adversary and I change places: he comes off best, although, as a matter of fact, he is in the wrong.
[Footnote 1: According to Diogenes Laertius, v., 28, Aristotle put Rhetoric and Dialectic together, as aiming at persuasion, [Greek: to pithanon]; and Analytic and Philosophy as aiming at truth. Aristotle does, indeed, distinguish between (1) Logic, or Analytic, as the theory or method of arriving at true or apodeictic conclusions; and (2) Dialectic as the method of arriving at conclusions that are accepted or pass current as true, [Greek: endoxa] probabilia; conclusions in regard to which it is not taken for granted that they are false, and also not taken for granted that they are true in themselves, since that is not the point. What is this but the art of being in the right, whether one has any reason for being so or not, in other words, the art of attaining the appearance of truth, regardless of its substance? That is, then, as I put it above.
Aristotle divides all conclusions into logical and dialectical, in the manner described, and then into eristical. (3) Eristic is the method by which the form of the conclusion is correct, but the premisses, the materials from which it is drawn, are not true, but only appear to be true. Finally (4) Sophistic is the method in which the form of the conclusion is false, although it seems correct. These three last properly belong to the art of Controversial Dialectic, as they have no objective truth in view, but only the appearance of it, and pay no regard to truth itself; that is to say, they aim at victory. Aristotle's book on Sophistic Conclusions was edited apart from the others, and at a later date. It was the last book of his Dialectic.]
If the reader asks how this is, I reply that it is simply the natural baseness of human nature. If human nature were not base, but thoroughly honourable, we should in every debate have no other aim than the discovery of truth; we should not in the least care whether the truth proved to be in favour of the opinion which we had begun by expressing, or of the opinion of our adversary. That we should regard as a matter of no moment, or, at any rate, of very secondary consequence; but, as things are, it is the main concern. Our innate vanity, which is particularly sensitive in reference to our intellectual powers, will not suffer us to allow that our first position was wrong and our adversary's right. The way out of this difficulty would be simply to take the trouble always to form a correct judgment. For this a man would have to think before he spoke. But, with most men, innate vanity is accompanied by loquacity and innate dishonesty. They speak before they think; and even though they may afterwards perceive that they are wrong, and that what they assert is false, they want it to seem the contrary. The interest in truth, which may be presumed to have been their only motive when they stated the proposition alleged to be true, now gives way to the interests of vanity: and so, for the sake of vanity, what is true must seem false, and what is false must seem true.
However, this very dishonesty, this persistence in a proposition which seems false even to ourselves, has something to be said for it. It often happens that we begin with the firm conviction of the truth of our statement; but our opponent's argument appears to refute it. Should we abandon our position at once, we may discover later on that we were right after all; the proof we offered was false, but nevertheless there was a proof for our statement which was true. The argument which would have been our salvation did not occur to us at the moment. Hence we make it a rule to attack a counter-argument, even though to all appearances it is true and forcible, in the belief that its truth is only superficial, and that in the course of the dispute another argument will occur to us by which we may upset it, or succeed in confirming the truth of our statement. In this way we are almost compelled to become dishonest; or, at any rate, the temptation to do so is very great. Thus it is that the weakness of our intellect and the perversity of our will lend each other mutual support; and that, generally, a disputant fights not for truth, but for his proposition, as though it were a battle pro aris et focis. He sets to work per fas et nefas; nay, as we have seen, he cannot easily do otherwise. As a rule, then, every man will insist on maintaining whatever he has said, even though for the moment he may consider it false or doubtful. 
[Footnote 1: Machiavelli recommends his Prince to make use of every moment that his neighbour is weak, in order to attack him; as otherwise his neighbour may do the same. If honour and fidelity prevailed in the world, it would be a different matter; but as these are qualities not to be expected, a man must not practise them himself, because he will meet with a bad return. It is just the same in a dispute: if I allow that my opponent is right as soon as he seems to be so, it is scarcely probable that he will do the same when the position is reversed; and as he acts wrongly, I am compelled to act wrongly too. It is easy to say that we must yield to truth, without any prepossession in favour of our own statements; but we cannot assume that our opponent will do it, and therefore we cannot do it either. Nay, if I were to abandon the position on which I had previously bestowed much thought, as soon as it appeared that he was right, it might easily happen that I might be misled by a momentary impression, and give up the truth in order to accept an error.]
To some extent every man is armed against such a procedure by his own cunning and villainy. He learns by daily experience, and thus comes to have his own natural Dialectic, just as he has his own natural Logic. But his Dialectic is by no means as safe a guide as his Logic. It is not so easy for any one to think or draw an inference contrary to the laws of Logic; false judgments are frequent, false conclusions very rare. A man cannot easily be deficient in natural Logic, but he may very easily be deficient in natural Dialectic, which is a gift apportioned in unequal measure. In so far natural Dialectic resembles the faculty of judgment, which differs in degree with every man; while reason, strictly speaking, is the same. For it often happens that in a matter in which a man is really in the right, he is confounded or refuted by merely superficial arguments; and if he emerges victorious from a contest, he owes it very often not so much to the correctness of his judgment in stating his proposition, as to the cunning and address with which he defended it.
Here, as in all other cases, the best gifts are born with a man; nevertheless, much may be done to make him a master of this art by practice, and also by a consideration of the tactics which may be used to defeat an opponent, or which he uses himself for a similar purpose. Therefore, even though Logic may be of no very real, practical use, Dialectic may certainly be so; and Aristotle, too, seems to me to have drawn up his Logic proper, or Analytic, as a foundation and preparation for his Dialectic, and to have made this his chief business. Logic is concerned with the mere form of propositions; Dialectic, with their contents or matter—in a word, with their substance. It was proper, therefore, to consider the general form of all propositions before proceeding to particulars.
Aristotle does not define the object of Dialectic as exactly as I have done it here; for while he allows that its principal object is disputation, he declares at the same time that it is also the discovery of truth.  Again, he says, later on, that if, from the philosophical point of view, propositions are dealt with according to their truth, Dialectic regards them according to their plausibility, or the measure in which they will win the approval and assent of others.  He is aware that the objective truth of a proposition must be distinguished and separated from the way in which it is pressed home, and approbation won for it; but he fails to draw a sufficiently sharp distinction between these two aspects of the matter, so as to reserve Dialectic for the latter alone.  The rules which he often gives for Dialectic contain some of those which properly belong to Logic; and hence it appears to me that he has not provided a clear solution of the problem.
[Footnote 1: Topica, bk. i., 2.]
[Footnote 2: Ib., 12.]
[Footnote 3: On the other hand, in his book De Sophisticis Elenchis, he takes too much trouble to separate Dialectic from Sophistic and Eristic, where the distinction is said to consist in this, that dialectical conclusions are true in their form and their contents, while sophistical and eristical conclusions are false.
Eristic so far differs from Sophistic that, while the master of Eristic aims at mere victory, the Sophist looks to the reputation, and with it, the monetary rewards which he will gain. But whether a proposition is true in respect of its contents is far too uncertain a matter to form the foundation of the distinction in question; and it is a matter on which the disputant least of all can arrive at certainty; nor is it disclosed in any very sure form even by the result of the disputation. Therefore, when Aristotle speaks of Dialectic, we must include in it Sophistic, Eristic, and Peirastic, and define it as "the art of getting the best of it in a dispute," in which, unquestionably, the safest plan is to be in the right to begin with; but this in itself is not enough in the existing disposition of mankind, and, on the other hand, with the weakness of the human intellect, it is not altogether necessary. Other expedients are required, which, just because they are unnecessary to the attainment of objective truth, may also be used when a man is objectively in the wrong; and whether or not this is the case, is hardly ever a matter of complete certainty.
I am of opinion, therefore, that a sharper distinction should be drawn between Dialectic and Logic than Aristotle has given us; that to Logic we should assign objective truth as far as it is merely formal, and that Dialectic should be confined to the art of gaining one's point, and contrarily, that Sophistic and Eristic should not be distinguished from Dialectic in Aristotle's fashion, since the difference which he draws rests on objective and material truth; and in regard to what this is, we cannot attain any clear certainty before discussion; but we are compelled, with Pilate, to ask, What is truth? For truth is in the depths, [Greek: en butho hae halaetheia] (a saying of Democritus, Diog. Laert., ix., 72). Two men often engage in a warm dispute, and then return to their homes each of the other's opinion, which he has exchanged for his own. It is easy to say that in every dispute we should have no other aim than the advancement of truth; but before dispute no one knows where it is, and through his opponent's arguments and his own a man is misled.]
We must always keep the subject of one branch of knowledge quite distinct from that of any other. To form a clear idea of the province of Dialectic, we must pay no attention to objective truth, which is an affair of Logic; we must regard it simply as the art of getting the best of it in a dispute, which, as we have seen, is all the easier if we are actually in the right. In itself Dialectic has nothing to do but to show how a man may defend himself against attacks of every kind, and especially against dishonest attacks; and, in the same fashion, how he may attack another man's statement without contradicting himself, or generally without being defeated. The discovery of objective truth must be separated from the art of winning acceptance for propositions; for objective truth is an entirely different matter: it is the business of sound judgment, reflection and experience, for which there is no special art.
Such, then, is the aim of Dialectic. It has been defined as the Logic of appearance; but the definition is a wrong one, as in that case it could only be used to repel false propositions. But even when a man has the right on his side, he needs Dialectic in order to defend and maintain it; he must know what the dishonest tricks are, in order to meet them; nay, he must often make use of them himself, so as to beat the enemy with his own weapons.
Accordingly, in a dialectical contest we must put objective truth aside, or, rather, we must regard it as an accidental circumstance, and look only to the defence of our own position and the refutation of our opponent's.
In following out the rules to this end, no respect should be paid to objective truth, because we usually do not know where the truth lies. As I have said, a man often does not himself know whether he is in the right or not; he often believes it, and is mistaken: both sides often believe it. Truth is in the depths. At the beginning of a contest each man believes, as a rule, that right is on his side; in the course of it, both become doubtful, and the truth is not determined or confirmed until the close.
Dialectic, then, need have nothing to do with truth, as little as the fencing master considers who is in the right when a dispute leads to a duel. Thrust and parry is the whole business. Dialectic is the art of intellectual fencing; and it is only when we so regard it that we can erect it into a branch of knowledge. For if we take purely objective truth as our aim, we are reduced to mere Logic; if we take the maintenance of false propositions, it is mere Sophistic; and in either case it would have to be assumed that we were aware of what was true and what was false; and it is seldom that we have any clear idea of the truth beforehand. The true conception of Dialectic is, then, that which we have formed: it is the art of intellectual fencing used for the purpose of getting the best of it in a dispute; and, although the name Eristic would be more suitable, it is more correct to call it controversial Dialectic, Dialectica eristica.
Dialectic in this sense of the word has no other aim but to reduce to a regular system and collect and exhibit the arts which most men employ when they observe, in a dispute, that truth is not on their side, and still attempt to gain the day. Hence, it would be very inexpedient to pay any regard to objective truth or its advancement in a science of Dialectic; since this is not done in that original and natural Dialectic innate in men, where they strive for nothing but victory. The science of Dialectic, in one sense of the word, is mainly concerned to tabulate and analyse dishonest stratagems, in order that in a real debate they may be at once recognised and defeated. It is for this very reason that Dialectic must admittedly take victory, and not objective truth, for its aim and purpose.
I am not aware that anything has been done in this direction, although I have made inquiries far and wide.  It is, therefore, an uncultivated soil. To accomplish our purpose, we must draw from our experience; we must observe how in the debates which often arise in our intercourse with our fellow-men this or that stratagem is employed by one side or the other. By finding out the common elements in tricks repeated in different forms, we shall be enabled to exhibit certain general stratagems which may be advantageous, as well for our own use, as for frustrating others if they use them.
[Footnote 1: Diogenes Laertes tells us that among the numerous writings on Rhetoric by Theophrastus, all of which have been lost, there was one entitled [Greek: Agonistikon taes peri tous eristikous gogous theorias.] That would have been just what we want.]
What follows is to be regarded as a first attempt.
THE BASIS OF ALL DIALECTIC.
First of all, we must consider the essential nature of every dispute: what it is that really takes place in it.
Our opponent has stated a thesis, or we ourselves,—it is all one. There are two modes of refuting it, and two courses that we may pursue.
I. The modes are (1) ad rem, (2) ad hominem or ex concessis. That is to say: We may show either that the proposition is not in accordance with the nature of things, i.e., with absolute, objective truth; or that it is inconsistent with other statements or admissions of our opponent, i.e., with truth as it appears to him. The latter mode of arguing a question produces only a relative conviction, and makes no difference whatever to the objective truth of the matter.
II. The two courses that we may pursue are (1) the direct, and (2) the indirect refutation. The direct attacks the reason for the thesis; the indirect, its results. The direct refutation shows that the thesis is not true; the indirect, that it cannot be true.
The direct course admits of a twofold procedure. Either we may show that the reasons for the statement are false (nego majorem, minorem); or we may admit the reasons or premisses, but show that the statement does not follow from them (nego consequentiam); that is, we attack the conclusion or form of the syllogism.
The direct refutation makes use either of the diversion or of the instance.
(a) The diversion.—We accept our opponent's proposition as true, and then show what follows from it when we bring it into connection with some other proposition acknowledged to be true. We use the two propositions as the premisses of a syllogism giving a conclusion which is manifestly false, as contradicting either the nature of things,  or other statements of our opponent himself; that is to say, the conclusion is false either ad rem or ad hominem.  Consequently, our opponent's proposition must have been false; for, while true premisses can give only a true conclusion, false premisses need not always give a false one.
[Footnote 1: If it is in direct contradiction with a perfectly undoubted, truth, we have reduced our opponent's position ad absurdum.]
[Footnote 2: Socrates, in Hippia Maj. et alias.]
(b) The instance, or the example to the contrary.—This consists in refuting the general proposition by direct reference to particular cases which are included in it in the way in which it is stated, but to which it does not apply, and by which it is therefore shown to be necessarily false.
Such is the framework or skeleton of all forms of disputation; for to this every kind of controversy may be ultimately reduced. The whole of a controversy may, however, actually proceed in the manner described, or only appear to do so; and it may be supported by genuine or spurious arguments. It is just because it is not easy to make out the truth in regard to this matter, that debates are so long and so obstinate.
Nor can we, in ordering the argument, separate actual from apparent truth, since even the disputants are not certain about it beforehand. Therefore I shall describe the various tricks or stratagems without regard to questions of objective truth or falsity; for that is a matter on which we have no assurance, and which cannot be determined previously. Moreover, in every disputation or argument on any subject we must agree about something; and by this, as a principle, we must be willing to judge the matter in question. We cannot argue with those who deny principles: Contra negantem principia non est disputandum.