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Lecture of Prof. Jim Delgrande

  • Contact informationIoannis Hatzilygeroudisemail:ihatz AT ceid.upatras.gr

02.12.2010, Room HL1, Dep. of Electrical and Computer Engineering


Prof. Jim Delgrande

Simon Fraser University

Vancouver, Canada




"What's in a Default: Thoughts on the Nature and Role of Defaults in Nonmonotonic Reasoning"



In this talk I consider the role and meaning of defaults in nonmonotonic reasoning (NMR). Defaults, that is, statements that expresses a condition of normally such as "adults are (normally) employed", are crucial in commonsense reasoning and arguably in artificial intelligence in general. The majority of research concerning defaults has focussed on (default) inference mechanisms, rather than representational issues concerning the meaning of a default. I suggest that, despite the very impressive formal work in the area, approaches to default reasoning are not unproblematic, and that it would be useful to (re)consider these approaches with respect to the phenomena that they are intended to model. To this end, I consider approaches to default reasoning from three points of view:

- with regards to their assertional force;

- with regards to individuals and instantiation; and

- with regards to whether an approach is best considered as analogous to a rule of inference or a weak version of the material conditional.

Following this, I argue that defaults of normality are best seen as statements in a naive scientific theory. Arguably a theory of the meaning of such defaults can be given in terms of a logic of weak conditionals, in which a default is treated as a counterfactual normative statement. From this vantage, nonmonotonic reasoning with such conditionals can be re-examined. To this end, the notion of relevant properties emerges as a key factor in drawing default conclusions about an individual. I suggest also that other phenomena, such as reasoning about norms, or deontic assertions, or counterfactuals may be addressed in a similar fashion.

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