Full Idea
The pretended direct contemplation of the mind by itself is a pure illusion. ...It is clear that, by an inevitable necessity, the human mind can observe all phenomena directly, except its own.
Gist of Idea
Introspection is pure illusion; we can obviously observe everything except ourselves
Source
Auguste Comte (Intro to Positive Philosophy [1830], Ch.1)
Book Reference
Comte,Auguste: 'Introduction to Positive Philosophy', ed/tr. Ferré,Frederick [Hackett 1988], p.20
A Reaction
I recently heard of a university psychology department which was seeking skilled introspectors to help with their researches. I take introspection to be very difficult, but partially possible. Read Proust.