 A second challenge comes from the so-called argument from non-veridical experience (Heck 2000). Suppose that a subject has a non-veridical experience as of a shade of color that does not exist in nature and that has never previous appeared in her experience. Then the subject cannot have an appropriate demonstrative concept, as there is nothing to fix the reference of that concept. For a response to this argument see Bengson, Grube, and Korman 2011.