Internal perception also fails as a method for scientific psychology. In the nature of the case, it must be casual and therefore unsystematic. It excludes all deliberate investigation, because as soon as it becomes aware of itself it turns into the self-observation that is open to all the well-known and valid criticisms. It is, indeed, the basis of a psychology of consciousness, in the sense that conscious processes must be perceived to be known, but for a science it is not enough.

p. 245 - "The history of introspection reconsidered." 1980