The philosophical basis of the theory of ideas is that there is an ideal form, and that the visible things on the earth are only the imperfect copies of the original. Human beings cannot normally attain to knowledge of this ideal form and are therefore limited to the world of senses. Plato gives two analogies. The first is the analogy of the prisoners in a cave with a fire burning behind them. The prisoners themselves cannot see the fire though they can see the shadow cast by them against the wall. The fire is symbolic of that knowledge as the supreme reality, of that ideal form, which the human beings cannot normally conceive of.

Another analogy is that of the Divided Line.