Methodological Simplicity in Expert System Construction the Case of Judgments and Reasoned Assumptions I Owe Much to Minsky for Inspiration on These Topics. I Also Thank

Probabilistic rules and their variants have recently supported several successful applications of expert systems, in spite of the diiculty of committing informants to particular conditional probabilities or \certainty factors," and in spite of the experimentally observed insensitivity of system performance to perturbations of the chosen values. Here we survey recent developments concerning reasoned assumptions which ooer hope for avoiding the practical elusiveness of probabilistic rules while retaining theoretical power, for basing systems on the information unhesitatingly gained from expert informants, and reconstructing the entailed degrees of belief later.