Our concern here is not to treat of empirical (e.g. optical) illusion, which occurs in the empirical use of otherwise correct rules of the understanding, and through which the faculty of judgment is misled through the influence of the imagination; rather, we have to do only with transcendental illusion, which influences principles whose use is not ever meant for experience, since in that case we would at least have a touchstone for their correctness, but which instead, contrary to all the warnings of criticism, carries us away beyond the empirical use of the categories, and holds out to us the semblance of extending the pure understanding.