Consciousness is the faculty by which internal phenomena become known to us. Let's examine the conditions of internal perception, just as we did those of external perception.

The first is that a modification of the self must occur. Every internal phenomenon is a form of knowledge, and for there to be knowledge, there must be something to know. This something is the psychic modification, the object of knowledge of consciousness, and the requirement that such an object exist corresponds to the first condition of external perception.

Second, this knowledge requires a subject – the self. The second condition of internal perception (which corresponds to the second condition of external perception) is thus the intervention of the self, which alone is capable of knowing. So internal perception has all the same conditions as external perception, except for the need for one of the senses to serve as an intermediary between subject and object.

It's been argued that some internal phenomena don't meet all the required conditions and thus can't be observed by consciousness. Leibniz, who first drew the attention of philosophers to this point, suggested that the internal world is composed of perceptions and apperceptions. We're fully conscious only of the latter. Leibniz's idea had significant implications and led to the formation of an entire doctrine whose most prominent representatives are Schopenhauer, author of The World as Will and Representation, and Hartmann, author of The Philosophy of the Unconscious.