rn to the question of
appropriateness later; for the present it is accuracy that I wish to
consider.

When we are viewing a man from the outside, it is not his beliefs,
but his bodily movements, that we can observe. His knowledge must be
inferred from his bodily movements, and especially from what he says
and writes. For the present we may ignore beliefs, and regard a man's
knowledge as actually consisting in what he says and does. That is to
say, we will construct, as far as possible, a purely behaviouristic
account of truth and falsehood.

If you ask a boy "What is twice two?" and the boy says "four," you take
that as prima facie evidence that the boy knows what twice two is. But
if you go on to ask what is twice three, twice four, twice five, and so
on, and the boy always answers "four," you come to the conclusion that
he knows nothing about it. Exactly similar remarks apply to scientific
instruments. I know a certain weather-cock which has the pessimistic
habit of always pointing to the north-east. If you were to see it first
on a cold March day, you would think it an excellent weather-cock; but
with the first warm day of spring your confidence would be shaken. The
boy and the weather-cock have the same defect: they do not vary their
response when the stimulus is varied. A good instrument, or a person
with much knowledge, will give different responses to stimuli which
differ in relevant ways. This is the first point in defining accuracy of
response.

We will now assume another boy, who also, when you first question him,
asserts that twice two is four. But with this boy, instead of asking him
different questions, you make a practice of asking him the same question
every day at breakfast. You find that he says five, or six, or seven, or
any other number at random, and you conclude that he also does not know
what twice two is, though by good luck he answered right the first time.
This boy is like a weather-cock which, instead of being stuck fast, is
always going round and round, changing without any change of wind. This
boy and weather-cock have the opposite defect to that of the previous
pair: they give different responses to stimuli which do not differ in
any relevant way.

In connection with vagueness in memory, we already had occasion to
consider the definition of accuracy. Omitting some of the niceties of
our previous discussion, we may say that an instrument is ACCURATE when
it avoids the defects of the two