Explaining 'explaining away.' (Special Section on Probabilistic Reasoning)

Explaining away is a common pattern of reasoning in which the confirmation of one cause of an observed or believed event reduces the need to invoke alternative causes. The opposite of explaining away also can occur, where the confirmation of one cause increases belief in another. We provide a general qualitative probabilistic analysis of intercausal reasoning and identify the property of the interaction among the causes (product synergy) that determines which form of reasoning is appropriate. Product synergy extends the qualitative probabilistic network (QPN) formalism to support qualitative intercausal inference about the directions of change in probabilistic belief. The intercausal relation also justifies Occam's razor, facilitating pruning in the search for likely diagnoses.