Parmenides posed this very question in the sixth century before Christ. He saw the world divided into pairs of opposites:
light/darkness, fineness/coarseness, warmth/cold, being/non-being. One half of the opposition he called positive (light, fineness, warmth, being), the other negative. We might find this division into positive and negative poles childishly simple except for one difficulty: which one is positive, weight or lightness?
Parmenides responded: lightness is positive, weight negative.Was he correct or not? That is the question. The only certainty is: the lightness/weight opposition is the most mysterious, most ambiguous of all.