In such cases, I shall suggest, the explanation is usually that the protagonists have not agreed on the criteria by which their propositions are to be tested.

At first sight this explanation may seem very unplausible. For it is difficult to imagine the protagonists failing to agree at least that an agreement about criteria is a necessary pre-requisite of fruitful discus­ sion. Indeed, if we were deliberately seeking a method of prolonging a controversy, we could hardly do better than refuse to clear up doubts as to whether such an agreement existed. On the other hand, it seems clear that disagreements about criteria are in fact likely to be the

unrecognized cause of intractable controversy at any rate in those cases where the protagonists are sufficiently expert reasoners to detect other possible causes, such as mistakes in deduction or inference. Moreover, it turns out-as we shall see-that not only disagreements but also agreements about criteria do often remain quite unacknowledged. In­ deed, the whole structure of serious discourse depends upon de facto, though not dejure, agreements to adopt unstated assumptions about the meanings of words, about what facts are obvious, about what is relevant, about what is sufficiently precise, etc. Most of these informal agreements are in practice unquestioningly accepted (otherwise discussion would be impossible)— so unquestioningly that they go unrecognized; for there are no snags to force the realization that agreements are involved at all. And it is because such unrecognized agreements are so common that unrecognized disagreements are also common: in so far as we fail to realize what it is that makes our discussions go smoothly, we fail also to realize what it is that makes them go wrong.