Why do the cave dwellers look at shadows [of a puppet show]? If Plato only wanted to express that the cave dwellers see derivative reality in a derivative light, the puppet show would be enough.
The cave dwellers , within their own consciousness, have no impediment to making a unification of reality and the unreal because, for them, this is a duck-rabbit distinction. The shadows are real because they move and act; the light is real because it makes the shadows possible. Without shadows, nothing is actual (only the emptiness of light) and without light nothing is actual, for then everything collapses into the shadow-being that hedges in the picture-show.
The cave dwellers are those who judge things by sensation. A sensation unifies presence and absence in reality. Touch fells both cold and heat as temperatures; the ear sees silence as making speech possible; the eye sees black as a color. So far as we judge by sensation, we tend to see the absence or privation of X not only as a reality, but even as a reality that makes X possible. What is creation, according to sensation? Isn’t it just a bright light exploding into a large black space? We can only visualize the big bang in the same way. Of itself, sensation has an indifference to the distinction between reality and its absence. The reality of inertia is a good illustration of this – so far as sense is concerned, there is no absolute difference between motion and rest. Both feel the same, look the same, and register no difference in measurement. The two have only a relative distinction and so there is no impediment to their absolute unification. What is true about motion and rest can be universalized even to the distinction between reality and its absence. What is a grand unified theory but the reducing of all distinctions of thing to relative distinctions that are to be overcome in an absolute unification?
So far as one decides to judge according to sensation (that is, so far as one is what we now call a scientist), the goal of knowledge will be unification of all phenomena, that is, reality. This unification will not be complete until it unifies even being and non-being. And so Democritus was the first scientist (as he is frequently recognized to be) because he was the first to posit a unification of being (atoms) and void (nothing). He was the first person to recognize in the principle of his inquiry what was the goal of the inquiry.