as we do
in physical science. Introspection, therefore, though it is one among
our sources of knowledge, is not, in isolation, in any degree more
trustworthy than "external" perception.

I come now to our second question: Does introspection give us materials
for the knowledge of relations other than those arrived at by reflecting
upon external perception? It might be contended that the essence of what
is "mental" consists of relations, such as knowing for example, and that
our knowledge concerning these essentially mental relations is entirely
derived from introspection. If "knowing" were an unanalysable relation,
this view would be incontrovertible, since clearly no such relation
forms part of the subject matter of physics. But it would seem that
"knowing" is really various relations, all of them complex. Therefore,
until they have been analysed, our present question must remain
unanswered I shall return to it at the end of the present course of
lectures.

LECTURE VII. THE DEFINITION OF PERCEPTION

In Lecture V we found reason to think that the ultimate constituents*
of the world do not have the characteristics of either mind or matter
as ordinarily understood: they are not solid persistent objects moving
through space, nor are they fragments of "consciousness." But we found
two ways of grouping particulars, one into "things" or "pieces of
matter," the other into series of "perspectives," each series being what
may be called a "biography." Before we can define either sensations
or images, it is necessary to consider this twofold classification
in somewhat greater detail, and to derive from it a definition of
perception. It should be said that, in so far as the classification
assumes the whole world of physics (including its unperceived portions),
it contains hypothetical elements. But we will not linger on the grounds
for admitting these, which belong to the philosophy of physics rather
than of psychology.

* When I speak of "ultimate constituents," I do not mean
necessarily such as are theoretically incapable of analysis,
but only such as, at present, we can see no means of
analysing. I speak of such constituents as "particulars," or
as "RELATIVE particulars" when I wish to emphasize the fact
that they may be themselves complex.

The physical classification of particulars collects together all those
that are aspects of one "thing." Given any one particular, it is
found often