Time is not an empirical concept. For neither co-existence nor succession would be perceived by us, if the representation of time did not exist as a foundation a priori.
When I have heard him talking to Papa during the sittings for the picture, I have sat wondering whether it could be that he has no belief in anybody else, because he has no belief in himself.
Upon the solution of this problem, or upon sufficient proof of the impossibility of synthetical knowledge a priori, depends the existence or downfall of metaphysics.