Axel Kohler points out that illusions understood as discrepancy between physical stimulus and percept (illusiond) have inspired progress in the history of experimental psychology. At first glance, this seems to be rather obvious. However, to define a discrepancy, one must have two comparable measures of the same thing. But this is often not the case. Take a given lamp that looks very dim to us during the day but blindingly bright at night. How bright is the stimulus really? We are unable to determine which of the two cases is more illusoryd. The perceiver does not normally notice the illusiond. Apparent motion, in contrast, which has been a very influential paradigm, is more than mere illusiond. By differentiating illusions into illusiond and illusionm, I am able to point out a strange inconsistency between the amount of error contained in an illusion and the perceptual conspicuity of this error. I argue that there are four varieties of discrepancy between physical stimulus and the related percept (illusiond). They can be grouped by the size of the discrepancy and the degree of awareness (see Figure 1). First, there are more or less subtle discrepancies that are ubiquitous and go unnoticed most of the time. In rare occasions, and usually triggered by a revealing piece of contradiction, they are noticed (illusionm). The second variety consists of very large discrepancies, such as found in many intuitive physics examples. For instance, a water surface may look fine even if it extends impossibly at a large angle from the horizontal. For instance, when asked to draw the surface level that water assumes in a tilted beaker, observers err as if they did not know that water remains parallel to the ground. And the more expert they become at avoiding spills, the larger the error becomes. Experienced bartenders produce the largest errors (see Hecht & Proffitt 1995). The perception of relational properties discussed in the target article falls into this category. Here the perceptual error can be enormous and still go unnoticed. Typically, we need to consult physics books and learn about a physical stimulus before we are convinced that our perception is erroneous. When conceiving of illusion as mere illusiond, we fail to honor the special case of illusionm. Illusionsd are ubiquitous. As a matter of fact, the core discipline of sensory psychology—psychophysics—can be thought of as the formal description of how a physical stimulus differs from its percept. It does so all the time. Illusionsm are a special case. They may warn the organism about where adjustments to the perceptual system are necessary in order to avoid potentially dangerous misjudgments. Or they may just be occasions where the perceptual system fails to suppress the perceptual process that has lost out in the competition to resolve the underspecification problem.