Using counterfactuals in knowledge-based programming

This paper adds counterfactuals to the framework of knowledge-based programs of Fagin, Halpern, Moses, and Vardi 3,4]. The use of counterfactuals is illustrated by designing a protocol in which an agent stops sending messages once it knows that it is safe to do so. Such behavior is difficult to capture in the original framework because it involves reasoning about counterfactual executions, including ones that are not consistent with the protocol. Attempts to formalize these notions without counterfactuals are shown to lead to rather counterintuitive behavior.Received: 15 November 2001, Accepted: 15 April 2004, Published online: 13 July 2004Joseph Y. Halpern: Work supported in part by NSF under grant IRI-96-25901, IIS-0090145, and CTC-0208535, by the Air Force Office of Scientific Research under grant F49620-96-1-0323 and F48620-02-1-0101, and by ONR under grants N00014-00-1-03-41, N00014-01-1-0795, and by the DoD Multidisciplinary University Research Initiative (MURI) program administered by the ONR under grant N00014-01-1-0795.)A preliminary version of this paper appeared in the Proceedings of the Seventh Conference on Theoretical Aspects of Rationality and Knowledge (TARK), 1998.