  The reasoning behind this shift in position sheds useful light on the motives which move Retentional theorists. Brentano believed that while we can never know with certainty that primary objects of consciousness exist, we can know with certainly that secondary objects exist, and are as they appear. Accordingly, if the secondary object of a proteraesthesis were a just-past act of sensing, we could know with certainty that we ourselves, as the subject of that act, existed a few moments ago. But Brentano held that we have no such knowledge: it is logically possible that we have only just now come into existence. By making the primary object the past tone, rather than the sensing of this tone, the unwanted consequence is averted. For further detail see Chisholm (1981), Albertazzi 1999 (59–65), Krauss (1976), Moran (1996).