Evidence from egocentric space is cited to support bicoding of navigation in three-dimensional space. Horizontal distances and space are processed differently from the vertical. Indeed, effector systems are compatible in horizontal space, but potentially incompatible (or chaotic) during transitions to vertical motion. Navigation involves changes in coordinates, and animal models of navigation indicate that time has an important role.

Näive Realists have recently proposed that illusions occur in circumstances that are ‘non-paradigmatic’ or with which we are insufficiently familiar. While this proposal may work for many of the illusions philosophers normally discuss, I argue in this paper that there are other illusions that do not fit this pattern. In particular, the vertical-horizontal illusion (VHI) occurs in circumstances that are both familiar and paradigmatic, while disappearing (or becoming attenuated) in more unusual circumstances.