Our habitual modes of thought, limited as they are by language, retain traces of this origin.

We cannot conceive any mental process except by some vague analogy to a physical process.
In other words, we can even now only think with perfect clearness when we are concerned with some object of common cognition.

Thus, the sphere of external sensation and of physical agencies furnishes us with the one type of thinkable thing or object of thought, and we habitually view subjective mental states as analogues of these.
Still, it may be said that these slight nascent errors are hardly worth naming, and the question would still appear to recur whether there are other fully developed errors deserving to rank along with illusions of sense.

Do we, it may be asked, ever actually mistake the quality, degree, or structure of our internal feelings in the manner hinted above, and if so, what is the range of such error?
In order to appreciate the risks of such error, let us compare the process of self-observation with that of external perception with respect to the difficulties in the way of accurate presentative knowledge.
_Misreading of Internal Feelings._ First of all, it is noteworthy that a state of consciousness at any one moment is an exceedingly complex thing.

It is made up of a mass of feelings and active impulses which often combine and blend in a most inextricable way.