DEGREES OF TESTABILITY
THEORIES may be more, or less, severely testable; that is to say, more, or less, easily falsifiable. The degree of their testability is of significance for the selection of theories.
In this chapter, I shall compare the various degrees of testability or falsifiability of theories through comparing the classes of their potential falsifiers. This investigation is quite independent of the question whether or not it is possible to distinguish in an absolute sense between falsifiable and non-falsifiable theories. Indeed one might say of the present chapter that it 'relativises' the requirement of falsifiability by showing falsifiability to be a matter of degree.
A theory is falsifiable, as we saw in section 23, if there exists at least one non-empty class of homotypic basic statements which are forbidden by it; that is, if the class of its potential falsifiers is not empty. If, as in section 23, we represent the class of all possible basic statements by a circular area, and the possible events by the radii of the circle, then we can say: At least one radius--or perhaps better, one narrow sector whose width may represent the fact that the event is to be 'observable'--must be incompatible with the theory and ruled out by it. One might then represent the potential falsifiers of various theories by sectors of various widths. And according to the greater and lesser width of the sectors ruled out by them, theories might then be said to have more, or less, potential falsifiers. (The question whether this 'more' or 'less' could be made at all precise will be left open for the moment.) It might then be said, further, that if the class of potential falsifiers of one theory is 'larger' than that of another, there will be