s obvious that there are grave difficulties in carrying out these
definitions, but we will not linger over them. We have to pass, as soon
as we can, from the physical standpoint, which we have been hitherto
adopting, to the standpoint of psychology, in which we make more use
of introspection in the first of the three senses discussed in the
preceding lecture.

But before making the transition, there are two points which must be
made clear. First: Everything outside my own personal biography is
outside my experience; therefore if anything can be known by me outside
my biography, it can only be known in one of two ways:

(1) By inference from things within my biography, or

(2) By some a priori principle independent of experience.

I do not myself believe that anything approaching certainty is to be
attained by either of these methods, and therefore whatever lies outside
my personal biography must be regarded, theoretically, as hypothesis.
The theoretical argument for adopting the hypothesis is that it
simplifies the statement of the laws according to which events happen
in our experience. But there is no very good ground for supposing that
a simple law is more likely to be true than a complicated law, though
there is good ground for assuming a simple law in scientific practice,
as a working hypothesis, if it explains the facts as well as another
which is less simple. Belief in the existence of things outside my own
biography exists antecedently to evidence, and can only be destroyed, if
at all, by a long course of philosophic doubt. For purposes of science,
it is justified practically by the simplification which it introduces
into the laws of physics. But from the standpoint of theoretical logic
it must be regarded as a prejudice, not as a well-grounded theory. With
this proviso, I propose to continue yielding to the prejudice.

The second point concerns the relating of our point of view to that
which regards sensations as caused by stimuli external to the nervous
system (or at least to the brain), and distinguishes images as
"centrally excited," i.e. due to causes in the brain which cannot be
traced back to anything affecting the sense-organs. It is clear that,
if our analysis of physical objects has been valid, this way of defining
sensations needs reinterpretation. It is also clear that we must be able
to find such a new interpretation if our theory is to be admissible.

To make the matter clear, we will take the