Protagoras said that man is the measure of all things. That is what is perceived to be the case by one man really is the case for him. By this definition, that which is must also be not, or is at the same time both good and bad, based on the perception of the viewer.
Being, for things that are, consists of their being perceived. “It is clear to you, being present, that I am sitting. To one who is not present, however, it is not clear that I am sitting. Therefore it is unclear whether I am sitting or not sitting.”