'Really' isn't a word we should use with simple confidence.  If a neutrino had a brain which had evolved in neutrino-sized ancestors, it would say that rock 'really' do consist mostly of empty space. We have brains that evolved in medium-sized ancestors, who couldn't walk through rocks, so our 'really' is a 'really' in which rocks are solid. "Really', for an animal, is whatever its brain needs it to be, in order to assist its survival. And because different species live in such different worlds, there will be a troubling variety of 'reallys'.

What we see of the real world is not the unvarnished real world but a model of the real world, regulated and adjusted by sense data - a model that is constructed so that it is useful for dealing with the real world. The nature of that model depends on the kind of animal we are. A flying animal needs a different kind of world model from a walking, a climbing or a swimming animal. Predators need a different kind of model from prey, even though their words necessarily overlap. A monkey's brain must have software capable of simulating a three-dimensional maze of branches and trunks. A water boatman's brain doesn't need 3D software, since it lives on the surface of the pond in an Edwin Abbott Flatland. A mole's software for constructing models of the world will be customized for underground use. A naked mole rat probably has world-representing software similar to a mole's. But a squirrel, although it is a rodent like the mole rat, probably has world-rendering software much more like a monkey's.

Richard Dawkins: The God delusion,  A Mariner Book, Houghton Mifflin Co., New York, 2008.