Let
us introduce two antithetical terms in order to avoid certain
elementary confusions: To the question “How
do you know that so-and-so is the case”, we
sometimes answer by giving “
criteria”
and sometimes by giving
“
symptoms”. If medical
science calls angina an inflammation caused by a
particular bacillus, and we ask in a particular case
“why do you say this man has got
angina?” then the answer “I
have found the bacillus so-and-so in his
blood” gives us the criterion, or what we may call the
defining criterion of angina. If on the other
hand the answer was, “His throat is
inflamed”, this might give us a symptom of
angina. I call “symptom”
a phenomenon of which experience has taught us that it coincided,
in some way or other, with the phenomenon which is our defining
criterion. Then to say, “A man has
angina” if this bacillus is found in him
is a tautology
40.
or it is a loose way of
stating the definition of
“angina”. But to say,
“A man has angina whenever he has an inflamed
throat” is to make a hypothesis.