on to this question. To ignore this question
would be like describing a chronometer without regard to its accuracy
as a time-keeper, or a thermometer without mentioning the fact that it
measures temperature.

Many difficult questions arise in connection with knowledge. It is
difficult to define knowledge, difficult to decide whether we have any
knowledge, and difficult, even if it is conceded that we sometimes have
knowledge to discover whether we can ever know that we have knowledge
in this or that particular case. I shall divide the discussion into four
parts:

I. We may regard knowledge, from a behaviourist standpoint, as exhibited
in a certain kind of response to the environment. This response must
have some characteristics which it shares with those of scientific
instruments, but must also have others that are peculiar to knowledge.
We shall find that this point of view is important, but not exhaustive
of the nature of knowledge.

II. We may hold that the beliefs that constitute knowledge are
distinguished from such as are erroneous or uncertain by properties
which are intrinsic either to single beliefs or to systems of beliefs,
being in either case discoverable without reference to outside fact.
Views of this kind have been widely held among philosophers, but we
shall find no reason to accept them.

III. We believe that some beliefs are true, and some false. This raises
the problem of VERIFIABILITY: are there any circumstances which can
justifiably give us an unusual degree of certainty that such and such a
belief is true? It is obvious that there are circumstances which in fact
cause a certainty of this sort, and we wish to learn what we can from
examining these circumstances.

IV. Finally, there is the formal problem of defining truth and
falsehood, and deriving the objective reference of a proposition from
the meanings of its component words.

We will consider these four problems in succession.

I. We may regard a human being as an instrument, which makes various
responses to various stimuli. If we observe these responses from
outside, we shall regard them as showing knowledge when they display
two characteristics, ACCURACY and APPROPRIATENESS. These two are quite
distinct, and even sometimes incompatible. If I am being pursued by
a tiger, accuracy is furthered by turning round to look at him, but
appropriateness by running away without making any search for
further knowledge of the beast. I shall retu