term applied to the epistemology of the 18th-century German philosopher Immanuel Kant (Kant, Immanuel), who held that the human self, or transcendental ego, constructs knowledge out of sense impressions and from universal concepts called categories that it imposes upon them. Kant's transcendentalism is set in contrast to those of two of his predecessors—the problematic idealism of René Descartes, who claimed that the existence of matter can be doubted, and the dogmatic idealism of George Berkeley, who flatly denied the existence of matter. Kant believed that ideas, the raw matter of knowledge, must somehow be due to realities existing independently of human minds; but he held that such things-in-themselves must remain forever unknown. Human knowledge cannot reach to them because knowledge can only arise in the course of synthesizing the ideas of sense.

Transcendental idealism has remained a significant strand in later philosophy, being perpetuated in various forms of Kantian and Neo-Kantian movements of thought.