The triad “thesis, antithesis, and synthesis” is attributed to Hegel, even though he does not mention it in his works. Some commentators have attributed this triad to Fichte, but they don’t point out the work where Fichte has written about it. Marx and Engels have used the triad in their materialistic conception of history—for instance, in Marx’s The Poverty of Philosophy. 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 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.
Since Plato and Aristotle have some fundamental disagreements, 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. Thomas Aquinas too was attempting to develop a synthesis. Thomistic philosophy is not Aristotelian—it is an attempt to resolve the tension between the works of Plato and Aristotle.