Full Idea
Even in our own case, we cannot draw the line separating our conscious mental states from our unconscious mental states.
Gist of Idea
We can't draw a clear line between conscious and unconscious
Source
Daniel C. Dennett (Consciousness Explained [1991], 14.2)
Book Reference
Dennett,Daniel C.: 'Consciousness Explained' [Penguin 1993], p.447
A Reaction
This strikes me as being a simple and self-evident truth, which anyone working on the brain takes for granted, but an awful lot of philosophers (stuck somewhere in the seventeenth century) can't seem to grasp.