Fichte developed the "Wissenschaftslehre" (Science of Knowledge), radicalizing Kant's transcendental idealism by grounding all knowledge and experience in a single self-positing principle - the absolute "I" posits itself and, as a necessary condition of self-consciousness, posits a "not-I" (the world of objects) as its limit. On this basis he developed an activity-centered account of selfhood and freedom as practical self-determination rather than passive representation.
Intellectual setting
Fichte's philosophy emerged directly from his early engagement with Kant's critical philosophy, and his 1798 essay linking God to the moral world order triggered the "atheism controversy" that cost him his Jena professorship; after Napoleon's occupation of Prussia he delivered his "Addresses to the German Nation" (1808) calling for national renewal through education.
Key ideas
The self-positing I
Fichte argued that self-consciousness is not a given fact but an act - the "I" constitutes itself by positing itself and, in the same act, positing a "not-I" as the objective world against which it defines its own boundaries and practical activity.
Freedom as practical self-determination
Fichte held that selfhood is fundamentally practical and active rather than a passive knowing subject, so that freedom consists in the self's ongoing striving to overcome the limits posed by the not-I through moral and practical activity.
Legacy
Fichte's activity-based idealism bridged Kant and the subsequent systems of Schelling and Hegel within German Idealism, and his "Addresses to the German Nation" became a foundational, and later contested, text in the history of German nationalism.