ABSTRACT
To say that (R) is logically equivalent to (R-) is to say that in every possible situation in which one is true so is the other. (R-), which is known as the ‘contrapositive’ of (R), has the same content as (R), at least in all respects which are relevant here. A generalization like (R) ‘All ravens are black’ is supported by finding confirming instances of black ravens. And accordingly it would seem that (R-) ‘Nothing which is not black is a raven’ is supported by confirming instances of things which are neither black nor ravens, like white pens. But a white pen does not seem to support ‘All ravens are black’. Most of the things we see are neither ravens nor black. Does each of them really add to our support for this generalization?
