In a seminal paper in 1983, Tversky and Kahneman showed that, with certain choices of content, human reasoners will consider a statement of the shape `A and B’ to be more probable than `A’. According to probability theory, this is a fallacy, for the probability of a conjunction of independent propositions has to be less than or equal to the individual probabilities of its conjuncts. In this talk I argue that the conjunction “fallacy” can in fact be understood as (mostly) rational thought acting on non-obviously pragmatically strengthened premises. I derive the observed ranking from a combination of Kratzer’s (1991) theory of modality (building on Lewis, 1981) and independent theories of epistemic (ignorance) implicatures. In a nutshell, I argue that the `A’ option is in fact interpreted as `A and possibly B and possibly not B’. I show that, if we assume that `B’ is considered a better possibility (in Kratzer’s sense) than `not

B’, then `A and B’ will also be a better possibility than `A and possibly not B’.

# Abstract for Mascarenhas’ talk

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