Full Idea
Introspection cannot be a form of perceiving, since that invariably involves sensory qualities, and no qualities occur in introspection other than those of the sensations and perceptions we introspect; there are no additional qualities.
Gist of Idea
Introspection is not perception, because there are no extra qualities apart from the mental events themselves
Source
David M. Rosenthal (Instrospection [1998])
Book Reference
'MIT Encyclopedia of Cognitive Sciences', ed/tr. various [www.? 1998], p.
A Reaction
This sounds pretty conclusive. Presumably introspection is best described as meta-thought rather than perception, which means that it involves beliefs and judgements, rather than new perceptual qualities. It has to be conceptual, and probably linguistic.