As we saw from David and Steffi Lewis' "Holes," sometimes ordinary objects are really strange. If one is sympathetic to nominalism (i.e. all there is are concrete objects) or materialism (i.e. all there is are material objects), then especially so. Task: given nominalism and materialism, give one example of something, which is ordinarily accepted to exist, but is very difficult to square with those views. Proviso: you can't use either mathematics or consciousness since those are go-to examples.
Associate Professor, Department of Philosophy, Lewis & Clark College