It may be added that the power of introspection is a comparatively new acquisition of the human race, and that, as it improves, the amount of error connected with its operation may reasonably be expected to become infinitesimal.
It is often supposed by those who undervalue the introspective method in psychology that there is a special difficulty in the detection of error in introspection, owing to the fact that the object of inspection is something individual and private, and not open to common scrutiny as the object of external perception.

Yet, while allowing a certain force to this objection I would point out, first of all, that even in sense-perception, what the individual mind is immediately certain of is its own sensations.

The relatively perfect certainty which finally attaches to the presentative side of sense-perception is precisely that which finally attaches to the results of introspection.
In the second place, it may be said that the contrast between the inner and the outer experience is much less than it seems.

In many cases our emotions are the direct result of a common external cause, and even when they are not thus attached to some present external circumstance, we are able, it is admitted, by the use of language, roughly to compare our individual feelings.

And such comparison is continually bringing to light the fact that there is a continuity in our mental structure, that our highest thoughts and emotions lead us back to our common sense-impressions, and that consequently, in spite of all individual differences of temperament and mental organization, our inner experience is in all its larger features a common experience.
I may add that this supposition of the common nature of our internal experience, as a whole, not only underlies the science of psychology, but is implied in the very process of detecting and correcting errors of introspection.