  First, having only one universal dimension and one that they are only represented to have at that, namely, distribution in time, the contents of inner sense cannot be quantified; thus no mathematical model of them is possible. Second, “the manifold of internal observation is separated only by mere thought”. That is to say, only the introspective observer distinguishes the items one from another; there are no real distinctions among the items themselves. Third, these items “cannot be kept separate” in a way that would allow us to connect them again “at will”, by which Kant presumably means, according to the dictates of our developing theory. Fourth, “another thinking subject [does not] submit to our investigations in such a way as to be conformable to our purposes” – the only thinking subject whose inner sense one can investigate is oneself. Finally and most damningly, “even the observation itself alters and distorts the state of the object observed” (1786, Ak. IV:471). Indeed, introspection can be bad for the health: it is a road to “mental illness” (‘Illuminism and Terrorism’, 1798, Ak. VII:133; see 161).