Once illusion is explained via sense-data, the illusion case cannot be distinguished from the delusion case by appealing to the presence of an environmental feature in the former and its absence in the latter
A perceptual case involving a distinctive sensory experience that is apt to give rise to an erroneous perceptual judgment about an actually existing environmental object (e.g., a stick that looks bent but is not)
sense-data(Argument from illusion in philosophy of perception)
The objects experienced in cases of illusion — things that have the features the perceiver takes themselves to be experiencing, but which are not material things or elements in the environment independent of the individual experiencer.
The distinction between sensory perception and judgmental acumen enables Austin to distinguish between central cases of illusion and central cases of delusion, and also to sketch explanations of what is going on in those cases that do not make appeal to sense-data. Austin takes the defender of (i) and (ii) to argue as follows. First, consider an illusion, for example a stick that looks bent but really isn’t. Such an illusion has two key features. First, it clearly involves a distinctive sensory