Broadly defined, Dialectic (Greek: Διαλεκτικη) is an exchange of propositions (theses) and counter-propositions (antitheses) resulting in a synthesis of the opposing assertions, or at least a qualitative transformation in the direction of the dialogue.  It is one of the three original liberal arts or trivium (the other members are rhetoric and grammar) in Western culture.  In ancient and medieval times, both rhetoric and dialectic were understood to aim at being persuasive (through dialogue).  The aim of the dialectical method, often known as dialectic or dialectics, is to try to resolve the disagreement through rational discussion.  One way -- the Socratic method -- is to show that a given hypothesis (with other admissions) leads to a contradiction; thus, forcing the withdrawal of the hypothesis as a candidate for truth.  Another way of trying to resolve a disagreement is by denying some presupposition of the contending thesis and antithesis; thereby moving to a third (syn)thesis.¹

"The history of the term dialectic would by itself constitute a considerable history of philosophy" (Barbara Cassin, ed., Vocabulaire européen des philosophies [Paris: Le Robert & Seuil, 2004], p.  306, trans.  M.K.  Jensen).  Briefly, the term dialectic owes much of its prestige to its role in the philosophy of Plato, where it figures as the logical method of philosophy in the Socratic dialectical method of cross-examination.  The term was given new life by Hegel, whose dialectically dynamic model of nature and history made it, as it were, a fundamental aspect of the nature of reality (instead of regarding the contradictions into which dialectics leads as a sign of the sterility of the dialectical method, as Kant tended to do in his Critique of Pure Reason).  In the mid-nineteenth century, the concept of dialectic was appropriated by Marx (see, for example, Das Kapital, published in 1867) and Engels and retooled in a non-idealist manner, becoming a crucial notion in their philosophy of dialectical materialism.  Thus this concept came, for a time, to play a prominent role on the world stage and in world history.  Today, dialectics can also refer to an understanding of how we can or should perceive the world (epistemology), an assertion of the interconnected, contradictory, and dynamic nature of the world outside our perception of it (ontology), or a method of presentation of ideas or conclusions.

In Plato's dialogues, Socrates typically argues by means of cross-examining someone else's assertions in order to draw out the inherent contradictions within the other's position.  For example, in the Euthyphro, Socrates asks Euthyphro to provide a definition of piety.  Euthyphro replies that the pious is that which is loved by the gods.  But, Socrates points out, the gods are quarrelsome and their quarrels, like human quarrels, concern objects of love or hatred.  Euthyphro consents that this is the case.  Therefore, Socrates reasons, at least one thing exists which certain gods love but other gods hate.  Again, Euthyphro consents.  Socrates concludes that if Euthyphro's definition of piety is true, then there must exist at least one thing which is both pious and impious (as it is both loved and hated by the gods) -- which, Euthyphro admits, is absurd.  This is also known as Socratic irony.

Hegel's dialectic, which he usually presented in a threefold manner, was vulgarized by Heinrich Moritz Chalybäus as comprising three dialectical stages of development: a thesis, giving rise to its reaction, an antithesis which contradicts or negates the thesis, and the tension between the two being resolved by means of a synthesis.  Hegel rarely used these terms himself: this model is not Hegelian but Fichtean.

In the Logic, for instance, Hegel describes a dialectic of existence: first, existence must be posited as pure Being (thesis); but pure Being, upon examination, is found to be indistinguishable from Nothing (antithesis); yet both Being and Nothing are united as Becoming (synthesis), when it is realized that what is coming into being is, at the same time, also returning to nothing (consider life: old organisms die as new organisms are created or born).

As in the Socratic dialectic, Hegel claimed to proceed by making implicit contradictions explicit: each stage of the process is the product of contradictions inherent or implicit in the preceding stage.  For Hegel, the whole of history is one tremendous dialectic, major stages of which chart a progression from self-alienation as slavery to self-unification and realization as the rational, constitutional state of free and equal citizens.  The Hegelian dialectic cannot be mechanically applied for any chosen thesis.  Critics argue that the selection of any antithesis, other than the logical negation of the thesis is subjective.  Then, if the logical negation is used as the antithesis, there is no rigorous way to derive a synthesis.  In practice, when an antithesis is selected to suit the user's subjective purpose, the resulting contradictions are rhetorical, not logical and the resulting synthesis not rigorously defensible against a multitude of other possible syntheses.  The problem with the Fichtean thesis -- antithesis -- synthesis model is that it implies that contradictions or negations come from outside of things.  Hegel's point is that they are inherent in and internal to things.  This conception of dialectics derives ultimately from Heraclitus.

Karl Marx and Friedrich Engels believed Hegel was standing on his head, and claimed to put him back on his feet, ridding Hegel's logic of its idealist orientation, and conceiving what is now known as materialist or Marxist dialectics.  In the work of Marx and Engels the dialectical approach to the study of history became intertwined with historical materialism, the school of thought exemplified by the works of Marx, Engels, and Lenin.  (Marx himself never referred to historical materialism.) Dialectical method came to be seen as the vital foundation for any Marxist politics, through the work of Karl Korsch, Georg Lukács and certain members of the Frankfurt School.  Under Stalin, Marxist dialectics developed into what was called diamat (short for dialectical materialism).  Some Soviet academics, most notably Evald Ilyenkov, continued with unorthodox philosophical studies of the Marxist dialectic, as did a number of thinkers in the West.

Marxism views dialectics as a framework for social development in which contradiction plays a central role.  This is perhaps best exemplified in Marx's Capital, which exemplifies two of Marx's central theories: that of the labor theory of value and the materialist conception of history.  At the heart of Marxist dialectics is the idea of contradiction, with class struggle playing the central role in social and political development, although Marx does identify other historically important contradictions, such as those between mental and manual labor and town and country.  Contradiction is the key to all other categories and principles of dialectical development: development by passage of quantitative change into qualitative ones, interruption of gradualness, leaps, negation of the initial moment of development and negation of this very negation, and repetition at a higher level of some of the features and aspects of the original state.

Many philosophers have offered critiques of dialectic, and it can even be said that hostility or receptivity to dialectics is one of the things that divides twentieth-century Anglo-American philosophy from the so-called "continental" tradition, a divide that only a few contemporary philosophers (among them Richard Rorty) have ventured to bridge.

One philosopher who has attacked the notion of dialectic again and again is Karl Popper.  In 1937 he wrote and delivered a paper entitled "What Is Dialectic?" in which he attacked the dialectical method for its willingness "to put up with contradictions" (Conjectures and Refutations: The Growth of Scientific Knowledge [New York: Basic Books, 1962], p.  316).  Popper concluded the essay with these words: "The whole development of dialectic should be a warning against the dangers inherent in philosophical system-building.  It should remind us that philosophy should not be made a basis for any sort of scientific system and that philosophers should be much more modest in their claims.  One task which they can fulfil quite usefully is the study of the critical methods of science" (Ibid., p.  335).

In chapter 12 of volume 2 of The Open Society and Its Enemies (1944; 5th rev.  ed., 1966) Popper unleashed a famous attack on Hegelian dialectics, in which he held Hegel's thought (unjustly, in the view of many philosophers, such as Walter Kaufmann) to some degree responsible for facilitating the rise of fascism in Europe by encouraging and justifying irrationalism.  In section 17 of his 1961 "addenda" to The Open Society, entitled "Facts, Standards, and Truth: A Further Criticism of Relativism," Popper refused to moderate his criticism of the Hegelian dialectic, arguing that it "played a major role in the downfall of the liberal movement in Germany, .  .  .  by contributing to historicism and to an identification of might and right, encouraged totalitarian modes of thought.  .  .  .  [and] undermined and eventually lowered the traditional standards of intellectual responsibility and honesty" (The Open Society and Its Enemies, 5th rev.  ed., vol.  2 [Princeton: Princeton University Press, 1966], p.  395).

¹ Musicologist Rose Rosengard Subotnick gives the following example: "A question posed by Fred Friendly on a PBS program entitled Hard Drugs, Hard Choices: The Crisis Beyond Our Borders (aired on WNET, Channel 13, in the New York area, February 26, 1990), illustrates that others, too, seem to find this dynamic enlightening: 'Are our lives so barren because we use drugs? Or do we use drugs because our lives are so barren?' The question is dialectical to the extent that it enables one to grasp the two opposed priorities as simultaneously valid."

from Wikipedia