The Strategy Behind Belief Revision: A Matter of Judging Probability or the Use of Mental Models? - eScholarship

The Strategy Behind Belief Revision: A Matter of Judging Probability or the Use of Mental Models? Ann G. Wolf (ann.g.wolf@psychol.uni-giessen.de) Department of Experimental Psychology, Otto-Behaghel-Strase 10F D-35394 Giesen, Germany Markus Knauff (markus.knauff@psychol.uni-giessen.de) Department of Experimental Psychology, Otto-Behaghel-Strase 10F D-35394 Giesen, Germany Ponens (MP) is the logic inference rule of the sort ‘if p then q’ where conclusion q is inferred from categorical statement p. Similarly, Modus Tollens (MT) is the logic inference rule of the sort ‘if p then q’ where conclusion not-p is inferred from the categorical statement not-q. They found that for both MP and MT inferences, the preferred change was disbelieving the conditional statement. They ascribed this finding to syntactical factors. Politzer and Carles (2001) investigated whether the conditional form of the first premise in MP and MT problems influences the relative entrenchment of the conditional and categorical premises by also using disjunctive and conjunctive syllogisms. In line with Elio and Pelletier’s (1997) findings, the conditional premise was revised more often than the categorical premise. Because the disjunctive and conjunctive major premises were also more questioned than their minor counterpart, the authors argue that is not the conditional nature of the major premise leading to these revision choices but its status quo of being the major premise, by which they mean that due to its compound nature its more likely to be the source of error. Elio (1997; second experiment) also found that the conditional statement was believed less than the categorical. The accounts put forward by Elio and Pelletier (1997) and Politzer and Carles (2001) for explaining the deeper entrenchment of the conditional are context independent (Hasson & Johnson-Laird, 2003). A point to remark here is that although they used non-basic inference problems and the accounts offered seem plausible within the context of their studies, the thematic content of their premises were science fictional. In real life, however, people reason about more mundane issues. Byrne and Walsh (2005) argued that relative entrenchment is a function of familiarity. They found that with problems in an unfamiliar domain, people tended to believe the categorical premise more. In contrast, with familiar problems people tended to believe the conditional more. The reason for this, they say, is that familiar conditionals may express rules and laws that are deeply entrenched and therefore people rather choose to revise the categorical fact. However, the familiar conditionals they used had a high intrinsic probability close to being factual of nature, therefore stripped from any subjective probability. Not all familiar statements express rules and laws and a familiar conditional need not necessarily be high in probability. With respect to the Abstract Research in the field of human reasoning has shown repeatedly that people find it reasonably easy to detect inconsistencies. The question that still remains is how people revise their beliefs to undo these inconsistencies. We report two experiments in which subjects had to make belief revision choices on modus ponens (MP) and modus tollens (MT) sets of problems that contained conditionals with different levels of probability. After the final statement of each set, which was stated to be true, they had to decide which of the first two statements they believed more. The results showed that with high and low probability problems, subjects revised their beliefs as a function of the probability of the conditional. However, when the conditional had a near 50% chance of occurring, the pattern of belief revision more resembled the mismatch principle as suggested by the mental model theory. Thus, people use different strategies when revising their beliefs depending on whether they are guided by semantics or mental models. Keywords: Belief revision, conditional probability, mental model theory, mismatch principle. Introduction Belief revision is the process by which one alters his or her belief state in the face of contradicting evidence. In the past, the topic of belief revision was reserved for logicians and philosophers to be concerned about (e.g. Gardenfors, 1988). Several opposing theories have been offered in the past to explain the underlying process of belief revision. The most long-standing theory of belief revision is that of epistemic entrenchment, developed within the field of artificial intelligence (e.g. Gardenfors, 1992). This notion holds that some pieces of information are more entrenched and are accordingly more easily believed than others. There have been some findings suggesting that epistemic entrenchment plays a role in belief revision. However, it is not clear exactly what defines epistemic entrenchment. What the current study aims to investigate is whether probability of conditionals serves as a kind of epistemic entrenchment in belief revision and to contrast this with the mental model theory of belief revision (Johnson-Laird, 2006; Johnson-Laird, Girotto, and Legrenzi, 2004). Elio and Pelletier (1997) were one of the first to incorporate the theory of belief revision into human deductive reasoning. They explored conditional reasoning wherein Modus