Georg Wilhelm Friedrich Hegel, lecturing at the University of Berlin from 1818 until his death from cholera in 1831, was the most-celebrated philosopher in Europe. Students traveled from across the German-speaking world to hear the system he claimed completed the development of philosophy. The Phenomenology of Spirit (1807) narrated the journey of consciousness from sense-certainty to absolute knowing; the Science of Logic (1812-16) reconstructed the categories of thought in their internal dialectical development; the Philosophy of Right (1820) applied the system to ethics and the modern state. Within fifty years of Hegel's death, philosophy had split into camps defined by their relationship to him: Hegelian idealists in Britain (F. H. Bradley), Marxist materialists across Europe (Karl Marx having stood Hegel on his feet by inverting idealism into materialism), and analytic philosophers who treated Hegel as the paradigm of what philosophy should not be.
Hegelian dialectic — popularly schematized as thesis-antithesis-synthesis — is more accurately the immanent self-development of concepts through internal contradiction. Hegel himself rarely used the triad; his own term is Aufhebung — sublation — retaining the three-fold meaning of cancel, preserve, and elevate. A concept, examined rigorously, reveals an internal tension; the resolution is not the destruction of the original but its transformation into a richer concept that preserves what was valid while cancelling what was contradictory. In the Phenomenology, the famous master-slave dialectic shows how the relation of recognition between two self-consciousnesses develops through mortal struggle, enslavement, and the slave's labor on nature into a structure where the slave is in some respects freer than the master. The master-slave passage has been one of the most-influential pieces of philosophical writing of the past two centuries — Marx reading it as proto-revolutionary, Kojève, Sartre, de Beauvoir, and Fanon deploying it in recognition-based theories of intersubjectivity and oppression. Absolute Idealism is Hegel's metaphysical position: reality is the self-development of Geist (Spirit) through history. Geist is the collective, social, and historical development of rational self-consciousness in human communities. History is the unfolding of freedom, with the modern constitutional state representing the highest realized form of Sittlichkeit (ethical life). The political reception split sharply: Right Hegelians read Hegel as defending the Christian-Prussian order; Left Hegelians (Bruno Bauer, Ludwig Feuerbach, the young Karl Marx, Friedrich Engels, Max Stirner) read him as providing the dialectical method that would transform the existing order. Marx wrote in 1873 that he had stood Hegel on his head by replacing idealist with materialist dialectic: the contradictions that drive history are contradictions in the material conditions of production, not in concepts.
Hegel's reception has gone through several substantial revivals. British Idealism (Bradley, Bosanquet, T. H. Green, McTaggart) was dominant in Anglophone universities ~1875-1910 before Russell's revolt against idealism organized the analytic tradition. In continental philosophy Hegel never went away: Alexandre Kojève's 1933-39 Paris lectures shaped a generation of French intellectuals; Theodor Adorno and the Frankfurt School worked through Hegelian dialectical method in their critique of instrumental reason. The Pittsburgh School (Sellars, McDowell, Brandom) produced an analytic neo-Hegelianism in the late twentieth century. Slavoj Žižek's extensive Hegel work has made Hegel central to post-Lacanian psychoanalytic and political theory. Recognition theory (Charles Taylor's Politics of Recognition, Axel Honneth's Struggle for Recognition, 1992) traces directly to the master-slave dialectic.