The quickest and most efficient way of coming to understand Idealist ontology is to go directly to the forefather of all Idealists - Plato. In The Republic, his major treatise on the ideal state, Plato has given us the famous Allegory of the Cave.l Imagine, he suggests, a group of people sitting in a dark cave chained down in such a way that they can look in only one direction, toward the expanse of wall on one side of the cave. Several yards behind them is an open fire providing light, and between the fire and where they are sitting is a raised runway along which figures move, casting their shadows upon the wall. The individuals, chained so that they face the wall, cannot see the fire or the figures, but only the shadows. Now, if we imagine them confined to this position for their entire lives, we must expect them to consider the shadows as real, genuinely existent beings. Not knowing anything else, having no three-dimensional beings to use for comparison, these prisoners in the cave would come to believe that what they saw before them represented true reality.
Now imagine that they are unchained and can turn around to see the fire and the figures which have occasioned the shadows. Certainly, says Plato, they would readjust their conception of reality, altering it to fit the new perceptual data that their eyes are now able to collect. Moving about the cave, they begin to get a sense of the three-dimensional character of their environment; and they conclude by thinking that they had been fooled all along, and that now they truly know what reality is.
But then imagine that they are led from the cave into the blinding brilliance of a noonday sun outside. Wouldnt they, asks Plato, be struck dumb by the complete impossibility of it? Wouldnt they turn away in complete bewilderment, not wishing to see the real truth of their world? Wouldnt they gradually retreat to their cave, preferring its more manageable environment to the fantastically incredible world of space and sunlight?
Well, then, suggests the allegory, here we humans are in our own cave - the world as we see it with our five senses. It looks real enough - rocks and trees and birds and men. But it is actually only a world of images, three-dimensional shadows of another, more genuinely real world - a world of pure ideas - standing behind this world we see and hear and touch. And this realm of pure ideas or pure mind is so absolute in its perfection, so superlatively complete in every way, as to possess an intensity beyond the reach of the human mind. Like the sun that blinds our eyes, the Absolute Mind completely overwhelms our feeble intellects; and we turn away from it, as we turn our eyes from the sun, bedazzled and injured by our attempt to perceive it. And so, preferring a more manageable and comfortable existence, even if less genuinely real, we retreat to our cave, the world of sense perception, permitting our intellects only occasional and fleeting glimpses of ultimate reality.
1Plato, The Republic, Book VII