 While Davidson never offers any substantive account of what the proper source of evidence is for the physical, he often invokes the notion of rationality as constraining mental ascription, and it is clear that whatever constrains physical ascription is supposed to pull in a different and potentially conflicting direction. One suggestive way of getting at Davidson’s idea here is through the traditional distinction between ‘normative’ and ‘descriptive’ concepts. When we look to uncover generalizations in the physically described world, what we find to follow from a certain set of physical conditions is a brute fact; our world is constituted in certain ways (in its governing laws) that we could imagine to be different. We may come to an empirical investigation with certain theoretical commitments that inevitably lead us to read the data in some ways rather than others; and there may indeed, as Davidson himself suggests, be constitutive a priori principles that govern very basic physical concepts such as ‘object’ and ‘event’ (Davidson 1970, 220; 1974b, 239; 1973a, 254; see  3.2  above). However, the constraints are far looser and allow for a wide variation in terms of empirical content—of what physical events and states can follow from others. This is contrasted with mental ascription, where the normative notion of rationality rules out the possibility of certain mental states following from others. This line of thinking is suggestive, but it is in need of considerable tightening.