Qualities imply what they are not

It seems true that no quality can exist of itself, that is, 'up-ness' cannot exist apart from 'down-ness', or 'red-ness' apart from 'green-ness'. Each quality possesses within itself something of all the qualities it is not, and this is especially so of opposites, which seem to announce their absent partner each time they are cited: the closed door is also not open, the heavy weight is also not light. It was Heraclitus who pointed out that the path up the hill is also the path down.