In other words, in the sphere of internal, as in that of external experience, the criterion of reality is the average and perfect, as distinguished from the particular variable and imperfect act of cognition.
We see, then, that error in the process of introspection is at least conceivable.

And now let us examine this process a little further, in order to find out what probabilities of error attach to it.
To begin with, then, an act of introspection, to be complete, clearly involves the apprehension of an internal feeling or idea as something mental and marked off from the region of external experience.

This distinct recognition of internal states of mind as such, in opposition to external impressions, is by no means easy, but presupposes a certain degree of intellectual culture, and a measure of the power of abstract attention.
_Confusion of Internal and External Experience._ Accordingly, we find that where this is wanting there is a manifest disposition to translate internal feelings into terms of external impressions.

In this way there may arise a slight amount of habitual and approximately constant error.

Not that the process approaches to one of hallucination; but only that the internal feelings are intuited as having a cause or origin analogous to that of sense-impressions.