In sum, the role of illusions in vision research has historically been very important. The beginnings of experimental psychology have attempted to measure illusionsd in terms of the discrepancy or error between physical stimulus and percept. I have attempted to show that this error is neither substantial enough to serve as a definition of illusion, nor particularly fascinating. Instead, illusionsd are as ubiquitous as they are typically unnoticed or indeterminate. In contrast, the cases that engage our imagination usually are manifest illusionsm. The latter can be defined even in cases where it is not meaningful to speak of illusiond.