Before the nineteenth century, idealism tended to be a train of thought in individual writers who posited the existence of unseen entities and claimed greater reality for them than the things in the world that could be sensed. Except for Plotinus (205–270) and other Neo-Platonists, idealism before the nineteenth century was limited to positing entities or structures that existed in a separate realm, independently of perceived reality, as humans perceive reality.

The nineteenth century idealists, in contrast, posited ideal entities and structures and also described their functions in ways that directly influenced the perceived world and events within it. A medical analogy is that before the nineteenth century, idealists were like philosophical “anatomists,” whereas in the nineteenth century, idealists also worked as philosophical “physiologists.” This last is especially true of Friedrich Hegel (1770–1831), although he could not have constructed his system without Immanuel Kant’s (1724–1804) work before him, and the directions in which Johann Gottlieb Fichte (1762–1814) and Friedrich Schelling (1775–1854) tried to take Kant’s work.