 On sense-datum theories, whenever you have a visual experience as if there is something present that is red, there really is something present that is red—a sense-datum. Now, in many cases of illusions and hallucination, there won’t be any ordinary red thing around for you to experience even if there seems to you to be a red thing present. So sense-data are either going to be mental objects or strange non-mental objects. Sense-datum theories usually depart from the following principle: