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Personnel Michiel Van Lambalgen (co-director) Frank Veltman (co-director) Description of the Proposed Research

The problem: the role of nonmonotonicity in cognition. The proposed programme investigates the interplay between language, reasoning and cognition, with particular emphasis on the role of nonmonotonicity. This phenomenon can be illustrated by the following example. When presented with the premisses (1) 'If Fred has to write an essay, he is in the library', and (2) 'Fred has to write an essay', most people would draw the conclusion that Fred is in the library. However, when the additional premiss (3) 'The library is closed', is supplied, people tend to withdraw this conclusion and to infer instead that Fred is not in the library. This simple example has been studied in cognitive psychology under the namèsuppression effect' (Byrne 1989), which reflects the fact that psychologists often regard this phenomenon as a failure to derive classical logical consequences. We think it is better described as an ability in nonmonotonic reasoning. Generally, we say that a relation of logical consequence is nonmonotonic if the addition of a new premiss to a given set of premises may make conclusions previously obtained invalid. Here is another, more recent, example from the psychological literature. Klauer et al. 2000 adduce experimental evidence for the claim that logical inference tasks (i.e. does conclusion C follow from premises A,B?) are typically not solved using the definition of validity of classical logic (`if the premisses are true, so is the conclusion'), but by the construction of a single model. Which model is constructed depends on whether the conclusion is believed or not. If the conclusion C is believed, then a person will construct a model of A,B,C; if this succeeds, then C follows, otherwise nothing follows. (If C is not believed, the person will construct a model of A,B, not-C.) This procedure is clearly nonmonotonic: adding another premiss D may make the set A,B,D,C inconsistent, so that C would, according to the above criterion, no longer follow from A,B,D. Nonmonotonic reasoning represents a natural way of thinking, and people are good at it. They have to be good at it, because planning in an uncertain environment (i.e. reasoning about change, events and actions) requires this type of reasoning. Speaking more formally, both monotonic and nonmonotonic reasoning start from the maxim: M) `assume only what is given in the premisses' but they implement (M) in different ways. Nonmonotonic reasoning takes (M) to mean: all assumptions beyond those required by the premisses …