 However, pure categoricalism falls prey to two sorts of difficulties. First, the contingency of causative roles has some unpalatable consequences: unbeknownst to us, two distinct properties can swap their roles; they can play the same role at the same time; the same role can be played by one property at one time and by another property at a later time; an “alien” property can replace a familiar one by playing its role (Black 2000). Secondly, and more generally, we are never able to know which properties play which roles, nor are we able to know the intrinsic nature of such properties. This consequence should be accepted with “Ramseyan humility” (Lewis 2009; see also Langton 1998 for a related, Kantian, sort of humility) or it should be countered in the same way as we decide to counter any broader version of skepticism (Schaffer 2005). On this issue, see also Whittle (2006); Locke (2009); Kelly (2013); Yates (2018b).