Some philosophers- e.g. Frank Cameron Jackson, Howard Robinson- contend that the sense-data theory of perception is the only theory that can make proper sense of the term ‘illusion’. Take, for instance, two birds in a field on a sunny day. One bird, a cardinal, is perched atop a tree branch relatively close to our position; the other, a Blue Jay, is enjoying a bird bath a little farther away. In our visual context, the cardinal will appear to be larger than the Blue Jay; however, the sense-data theorist argues, the cardinal is not larger than the Blue Jay, and thus what we are directly aware of in perception are not physical objects, but rather, sense-data.
It may be asked, however, that if what I perceive (am directly aware of) are and can only be sense-data, by what measure may I determine perceptual experiences to be in ‘error’ or ‘faulty’? In the above example, I submit that the sense-data theorist is not permitted to judge the cardinal to be “not larger than the Blue Jay”. At best he may assert that, in this particular perceptual context, the cardinal is larger than the Blue Jay. He will also have to permit other, more intuitively problematic, assertions, such as, “The Earth is larger than the Sun.”
In essence, the sense-data theorist must permit all perceptual experiences equal validity in any given, immediate context, and is thus unable to cogently use terms such as “illusion”, “delusion”, and “hallucination”.