The pursuit of things in an ideal form. In philosophy, any of several theories (principally of Berkely, Hegel, and Kant) which hold that reality has no objective existence but is produced in some way by the mind.Quote: "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." Henry David Thoreau (1817-62), U.S. philosopher, author, naturalist. Walden, "Conclusion" (1854).Also see idealization and Surrealism.