Ramseyan Humility, scepticism and grasp

In ‘Ramseyan Humility’ David Lewis argues that a particular view about fundamental properties, quidditism, leads to the position that we are irredeemably ignorant of the identities of fundamental properties. We are ignorant of the identities of fundamental properties since we can never know which properties play which causal roles, and we have no other way of identifying fundamental properties other than by the causal roles they play. It has been suggested in the philosophical literature that Lewis’ argument for Humility is merely an instance of traditional scepticism, to which traditional responses to scepticism are applicable. I agree that in ‘Ramseyan Humility’ Lewis does present an argument to which it is appropriate to consider the applicability of responses to traditional scepticism—he argues that we irredeemably lack the evidence to rule out possibilities in which different properties occupy the causal roles described by our best physical theory. And prima facie this is just the kind of argument responses to traditional scepticism are designed to tackle. However, I will argue that Lewis bolsters this argument with a second. This second argument serves to deepen Lewis’ case and cannot be met with a response to traditional scepticism. For Lewis argues that not only do we lack evidence for which properties play which roles, we lack the ability to grasp any such proposition about role-occupancy. And if we cannot grasp any such proposition we cannot know it.