German idealism is often seen as the completion of the subjectivization of knowledge and reality begun by Descartes.

Not so, says Frederick Beiser in his massive 2002 history of German idealism (Harvard): “In fundamental respects it is more accurate to say the exact opposite: that the development of German idealism is not the culmination but the nemesis of the Cartesian tradition. Explicitly and emphatically, the German idealists criticized some of the central assumptions of that tradition: that self-consciousness is certain and given; that we know ourselves with more certainty than objects in space; that knowledge is the result of contemplation rather than action; that the bearers of meaning are ideas; and, at least after Kant, that we know ourselves apart from and prior to others. This critique of the Cartesian legacy begins with the early Kant; it grows in intensity with the first Kritik ; and it comes to a climax with Fichte, Schelling, and Hegel.

Idealism went through various phases because earlier formulations had not, in the opinion of successors, established a secure foundation for realism: “Kant rejected Leibniz’s and Berkeley’s empirical idealism because it had made the existence of objects in space a mere illusion; Fichte became disappointed with Kant’s transcendental idealism because he had not provided a sufficient basis for his empirical realism; Holderlin, Schlegel, Novalis, and Schelling broke with Fichte’s ethical idealism because it still trapped the ego inside the circle of consciousness.”

Idealism was an effort to reconcile the epistemological necessity of subject-object identity with the experience of subject-object dualism. To know at all, there must be some sort of identity between the knowing subject and the object of knowledge. But this identity is destabilized by the recognition that knowing subject and known object are distinct in the real world. The issue is “how to find a principle of subject-object identity that could surmount but also explain the subject-object dualism of ordinary experience.” Beiser finds Hegel’s summary apt: The problem of German idealism was to discover “the identity of identity and nonidentity.”