popper-logic-scientific-discovery

appendix *ix 411
corroboration or acceptability cannot be a probability as one of the more
interesting findings of the philosophy of knowledge. It can be put very
simply like this. A report of the result of testing a theory can be
summed up by an appraisal. This can take the form of assigning some
degree of corroboration to the theory. But it can never take the form
of assigning to it a degree of probability; for the probability of a statement
(given some test statements) simply does not express an appraisal of the severity of the
tests a theory has passed, or of the manner in which it has passed these tests. The main
reason for this is that the content of a theory—which is the same as its
improbability—determines its testability and its corroborability.
I believe that these two ideas—content and degree of corroboration—are the
most important logical tools developed in my book. 8
So much by way of introduction. In the three notes which follow
here I have left the word ‘confirmation’ even where I should now only
write ‘corroboration’. I have also left ‘P(x)’ where I now usually write
‘p(x)’. But I have corrected some misprints; 9 and I have added a few
footnotes, preceded by stars, and also two new points, *13 and *14, to
the end of the Third Note.
8
As far as I am aware, the recognition of the significance of the empirical content or assertive
power of a theory; the suggestion that this content increases with the class of the potential
falsifiers of the theory—that is to say, the states of affairs which it forbids, or excludes
(see sections 23, and 31); and the idea that content may be measured by the improbability
of the theory, were not taken by me from any other source but were ‘all my own
work’. I was therefore surprised when I read in Carnap’s Introduction to Semantics, 1942, p.
151, in connection with his definition of ‘content’: ‘. . . the assertive power of a sentence
consists in its excluding certain states of affairs (Wittgenstein); the more it excludes, the
more it asserts.’ I wrote to Carnap, asking for details and reminding him of certain
relevant passages in my book. In his reply he said that his reference to Wittgenstein was
due to an error of memory, and that he actually had a passage from my book in mind;
and he repeated this correction in his Logical Foundations of Probability, 1950, p. 406. I
mention this here because in a number of papers published since 1942, the idea of
content—in the sense of empirical or informative content—has been attributed, without
definite reference, to Wittgenstein, or to Carnap, and sometimes to Wittgenstein and
myself. But I should not like anybody to think that I have taken it without acknowledgement
from Wittgenstein or anybody else: as a student of the history of ideas, I think that
it is quite important to refer to one’s sources. (See also my discussion in section 35 of
the distinction between logical content and empirical content, with references to Carnap in
footnotes 1 and 2.)
9 I have also, of course, incorporated the corrections mentioned in B.J.P.S. 5, pp. 334
and 359.