Karttunen & Peters connect their theory to the earlier attempts to define pragmatic presupposition, along the following lines: co-operative participants have the obligation to “organize their contributions in such a way that the conventional implicata of the sentence uttered are already part of the common ground at the time of utterance” (1975: 269). As we have seen, this is too strong a constraint, and it will be sufficient to require that the so-called conventional implicata are consistent with the common ground.
There are a number of substantial problems for this theory. It is formulated specifically to deal with the problems of projection that we reviewed above, and the solutions offered are what we may call ‘engineering solutions’ — i.e, whatever is required in the way of formal apparatus is simply built into the compositional process of sentence construction. In order to handle the intricacies of the projection problem, therefore, the details of the engineering must become increasingly complicated. It is possible, for example, to show that the latest formulation does not in fact handle some of the more intractable cases. For example, the filtering rule for conditionals we sketched in (173) is identical to the rule for conjunctions, and so the rule for conjunctions incorrectly predicts that (176) has the presupposition (177) (this counter-example is drawn from the substantial set assembled in Gazdar, I979a: Io8-I9):
(176) It is possible that John has children and it is possible that his children are away
(177) John has children
This happens because the filtering rule in (173) will predict that the presuppositions of (176) are (or at least include) those in (178):
(178) John exists and if it is possible that John has children then John has children
But since the antecedent of the conditional in (178) is entailed by (176), (176) plus the conditional entails (177). So it is predicted, incorrectly, that (176) will have (177) as a presupposition. Since the solutions are simply of an engineering sort, it remains open to Karttunen & Peters to try to re-tool the solutions to cope with the known counter-examples of this sort. Rather more troublesome is the evidence that the proposed filtering constraints are asymmetrical in the way that (137) is above — this makes it impossible to account for the filtering in (179) (drawn from Wilson, 1975) where the consequent entails what the antecedent presupposes, namely (180):
(179) If Nixon knows the war is over, the war is over
(180) The war is over