Timaeus by Plato. Socrates.
One, two, three; but where, my dear Timaeus, is the fourth of those who were yesterday my guests and are to be my entertainers to-day? Timaeus. He has been taken ill, Socrates; for he would not willingly have been absent from this gathering. Soc. Allegory of the Cave. Plato realizes that the general run of humankind can think, and speak, etc., without (so far as they acknowledge) any awareness of his realm of Forms.
The allegory of the cave is supposed to explain this. In the allegory, Plato likens people untutored in the Theory of Forms to prisoners chained in a cave, unable to turn their heads. All they can see is the wall of the cave. Behind them burns a fire. Between the fire and the prisoners there is a parapet, along which puppeteers can walk. A Finger, Two Dots Then Me.