"There is something such that it is not the case that it flies"

This Grice holds is a true thing to say of Pegasus.

On the other hand, to say:

"There is something such that it is SOMETHING which does not fly" is not.

In Grice's term:

"The degree of appeal which [my natural deduction system]

will have as a model of natural discourse will depend on

one's willingness to distinguish (for example)"

between (a) and (b)

(a) There is something such that it is not the case that it flies.
(b) There is something such that it is not something which does not fly.

"and to hold that the former is justified

by its being false that Pegasus flies, while

the latter can be justified only by its

being true of some actual object

that it does not fly."