If once we agree that we can know of nothing but unrelated thought pictures, because our consciousness is not in a position to relate these pictures to a unifying reality, then we have no right to ascribe, with Descartes and his school, an objective reality to the self. Even though the self may appear to us as the unifying agent among our thoughts, it must itself be a mental picture among mental pictures ; and man can have no knowledge of any permanent reality outside this fluctuating picture-realm. So, with Hume, the onlooker-consciousness came to experience its own utter inability to achieve a knowledge of the objective existence either of a material world be - behind all external phenomena, or of a spiritual self behind all the details of its own internal content.