 How can we perceive absences? They seem causally inert and so not the sort of thing that we could check empirically. Negative truths seem redundant; there are no more truths than those entailed by the conjunction of all positive truths. The negative truths seem psychological; we only assert negative truths to express a frustrated expectation. When Jean Paul Sartre (1969, 41) arrives late for his appointment with Pierre at the cafe, he sees the absence of Pierre but not the absence of the Duke of Wellington.