Deductive reasoning from uncertain conditionals.

This paper begins with a review of the literature on plausible reasoning with deductive arguments containing a conditional premise. There is concurring evidence that people presented with valid conditional arguments such as Modus Ponens and Modus Tollens generally do not endorse the conclusion, but rather find it uncertain, in case (1) the plausibility of the major conditional premise is debatable, (2) the major conditional premise is formulated in frequentist or probabilistic terms, or (3) an additional premise introduces uncertainty about the major conditional premise. This third situation gives rise to non-monotonic effects by a mechanism that can be characterized as follows: the reasoner is invited to doubt the major conditional premise by doubting the satisfaction of a tacit condition, which is necessary for the consequent to occur. Three experiments are presented. The first two aim to generalize the latter result using various types of conditionals and the last shows that performance in conditional reasoning is significantly affected by the representation of the task. This third point is discussed along with various other issues: we propose a pragmatic account of how the tacit conditions mentioned earlier are treated in plausible reasoning; the relationship of this account with the conditional probability view on conditional sentences is examined; an application of the same account to the Suppression Effect (Byrne, 1989) is proposed and compared with the counter-example availability explanation; and finally, some suggestions on how uncertainty could be implemented in a mental logic system are presented.