Like we disregard the sensations within a dream as unreal, the sages of the Upanishads discarded everything that was in a constant process of change as unreal. Their principle was _neti, neti atman_: “this is not the self, that is not the self”. When you dig into your own personality deeper, you find layers of perceptions, thoughts, emotions, drives, and memories. The sages found that none of these are permanent and ignored these attributes in their pursuit of true nature of the universe.