early 15c., "pertaining to an archetype or model," from Late Latin idealis "existing in idea," from Latin idea in the Platonic sense (see idea). Senses "conceived as perfect; existing only in idea," are from 1610s.
It seems even incredible, that any Idealist in any age could forget himself so far as to run his head against a post, merely because he found in his system, that no external world does exist, and that therefore nothing could be without to hurt him. [F.A. Nitsch, "A General and Introductory View of Professor Kant's Principles," 1796]
Earlier still, "one who holds doctrines of philosophical idealism" (1701).
1796 in the abstract metaphysical sense "belief that reality is made up only of ideas," from ideal (adj.) + -ism. Probably formed on model of French idéalisme. Meaning "tendency to represent things in an ideal form" is from 1829. Meaning "pursuit of the ideal, a striving after the perfect state" (of truth, purity, justice, etc.).
In the philosophical sense the Germans have refined it into absolute (Hegel), subjective (Fichte), objective (von Schelling), and transcendental (Kant).
That the glory of this world in the end is appearance leaves the world more glorious, if we feel it is a show of some fuller splendour : but the sensuous curtain is a deception and a cheat, if it hides some colourless movement of atoms, some spectral woof of impalpable abstractions, or unearthly ballet of bloodless categories. Though dragged to such conclusions, we can not embrace them. Our principles may be true, but they are not reality. They no more make that Whole which commands our devotion, than some shredded dissection of human tatters is that warm and breathing beauty of flesh which our hearts found delightful. [F.H. Bradley, "Principles of Logic," 1883]