If perception is constructed, what keeps perception from becoming mere hallucination unlinked to world events? The visual system has evolved two strategies to anchor itself and correct its errors. One involves completing missing information on the basis of knowledge about what most likely exists in the scene. For example, the visual system fills in information only in cases where it might be responsible for the data loss. The other strategy involves exploiting the physical stability of the environment as a reference frame with respect to which the eyes and body can move.