hearing which cannot
distinguish between them is inaccurate or vague in this respect.

Precision and vagueness in thought, as in perception, depend upon the
degree of difference between responses to more or less similar stimuli.
In the case of thought, the response does not follow immediately upon
the sensational stimulus, but that makes no difference as regards our
present question. Thus to revert to memory: A memory is "vague" when
it is appropriate to many different occurrences: for instance, "I met a
man" is vague, since any man would verify it. A memory is "precise" when
the occurrences that would verify it are narrowly circumscribed: for
instance, "I met Jones" is precise as compared to "I met a man." A
memory is "accurate" when it is both precise and true, i.e. in the above
instance, if it was Jones I met. It is precise even if it is false,
provided some very definite occurrence would have been required to make
it true.

It follows from what has been said that a vague thought has more
likelihood of being true than a precise one. To try and hit an object
with a vague thought is like trying to hit the bull's eye with a lump of
putty: when the putty reaches the target, it flattens out all over it,
and probably covers the bull's eye along with the rest. To try and hit
an object with a precise thought is like trying to hit the bull's
eye with a bullet. The advantage of the precise thought is that it
distinguishes between the bull's eye and the rest of the target. For
example, if the whole target is represented by the fungus family and the
bull's eye by mushrooms, a vague thought which can only hit the target
as a whole is not much use from a culinary point of view. And when I
merely remember that I met a man, my memory may be very inadequate to my
practical requirements, since it may make a great difference whether I
met Brown or Jones. The memory "I met Jones" is relatively precise. It
is accurate if I met Jones, inaccurate if I met Brown, but precise in
either case as against the mere recollection that I met a man.

The distinction between accuracy and precision is however, not
fundamental. We may omit precision from out thoughts and confine
ourselves to the distinction between accuracy and vagueness. We may then
set up the following definitions:

An instrument is "reliable" with respect to a given set of stimuli when
to stimuli which are not relevantly different it gives always responses
which are not relevantl