The triad “thesis, antithesis, and synthesis” is attributed to Hegel. But he does not mention this triad in his works. Some commentators have attributed this triad to Fichte, but they don’t point out the work where Fichte has deployed it. Karl Marx and Friedrich Engels have used the triad in their materialistic conception of history—for instance, in Marx’s The Poverty of Philosophy. But the Marxist version of the triad is an outcome of their own understanding of the philosophy of Hegel and Fichte.
The origin of the triad, however, can be traced to Ancient Greece—to the Classical Philosophy of Socrates and Plato, and Aristotle. The Platonic dialogues follow a dialectical form: An argument (thesis) is followed by a counterargument (antithesis). Either the counterargument cancels out the argument or it leads to the development of a position which consists of a “synthesis” of the opposing arguments and represents an improvement in the position taken by the argument and the counterargument. Aristotle has said that rhetoric is a counterpart of dialectics.
Plato and Aristotle agree on several points—but they have several major differences too. Platonism can be seen as the thesis, and Aristotelianism as the antithesis. The process of developing a synthesis between Platonism and Aristotelianism began with the Hellenistic Neo-Platonists and the philosophers in Ancient Rome, and was finally accomplished by Thomas Aquinas in the 13th century. The philosophy of Aquinas is not pure Aristotelian—it is an attempt to resolve the tension between the works of Plato and Aristotle.