s similar to sensations as regards extent and
duration. As against the view that introspection reveals a mental world
radically different from sensations, I propose to argue that thoughts,
beliefs, desires, pleasures, pains and emotions are all built up out
of sensations and images alone, and that there is reason to think that
images do not differ from sensations in their intrinsic character. We
thus effect a mutual rapprochement of mind and matter, and reduce the
ultimate data of introspection (in our second sense) to images alone. On
this third view of the meaning of introspection, therefore, our decision
is wholly against it.

There remain two points to be considered concerning introspection. The
first is as to how far it is trustworthy; the second is as to whether,
even granting that it reveals no radically different STUFF from that
revealed by what might be called external perception, it may not reveal
different RELATIONS, and thus acquire almost as much importance as is
traditionally assigned to it.

To begin with the trustworthiness of introspection. It is common among
certain schools to regard the knowledge of our own mental processes as
incomparably more certain than our knowledge of the "external" world;
this view is to be found in the British philosophy which descends from
Hume, and is present, somewhat veiled, in Kant and his followers.
There seems no reason whatever to accept this view. Our spontaneous,
unsophisticated beliefs, whether as to ourselves or as to the outer
world, are always extremely rash and very liable to error. The
acquisition of caution is equally necessary and equally difficult in
both directions. Not only are we often un aware of entertaining a
belief or desire which exists in us; we are often actually mistaken. The
fallibility of introspection as regards what we desire is made evident
by psycho-analysis; its fallibility as to what we know is easily
demonstrated. An autobiography, when confronted by a careful editor
with documentary evidence, is usually found to be full of obviously
inadvertent errors. Any of us confronted by a forgotten letter written
some years ago will be astonished to find how much more foolish our
opinions were than we had remembered them as being. And as to the
analysis of our mental operations--believing, desiring, willing, or what
not--introspection unaided gives very little help: it is necessary to
construct hypotheses and test them by their consequences, just