Recent research in decision making reported a description-experience (DE) gap: opposite risky choices when decisions are made from descriptions (gambles in which probability distributions and outcomes are explicitly stated) and when decisions are made from experience (the knowledge of the gambles is obtained by sampling outcomes from unknown probability distributions before making a choice). The DE gap has been reported in gambles commonly involving a risky option (outcomes drawn from a fixed probability distribution) and a safe option (probability of the outcome is 1), or in gambles involving two risky options. Here, we extend the study of the DE gap to gambles in which people choose between a risky option and an ambiguous option (with two nested probability distributions, where the eventgeneration mechanism is more opaque than that in the risky option). We report empirical evidence and show a DE gap in gambles involving risky and ambiguous options. Participants' choices are influenced by the information format and by the ambiguous option: participants are ambiguity-seeking in experience and ambiguity-averse in description in problems involving both gains and losses. In order to find reasons for our results, we investigate participants' sampling behavior, and this analysis indicates choices according to a cognitive model of experiential decisions (instance-based learning). In experience, participants have small sample sizes, and participants choose options where high outcomes are experienced more frequently than expected. We discuss the implications of our results for the psychology of decision making in complex environments.

Using a slight generalization, due to Palmgren, of sheaf semantics, we present a term-model construction that assigns a model to any first-order intuitionistic theory. A modification of this construction then assigns a nonstandard model to any theory of arithmetic, enabling us to reproduce conservation results of Moerdijk and Palmgren for nonstandard Heyting arithmetic. Internalizing the construction allows us to strengthen these results with additional transfer rules; we then show that even trivial transfer axioms or minor strengthenings of these rules destroy conservativity over HA. The analysis also shows that nonstandard HA has neither the disjunction property nor the explicit definability property. Finally, careful attention to the complexity of our definitions allows us to show that a certain weak fragment of intuitionistic nonstandard arithmetic is conservative over primitive recursive arithmetic.

Daniel Ellsberg presented in Ellsberg (The Quarterly Journal of Economics 75: 1961) various examples questioning the thesis that decision making under uncertainty can be reduced to decision making under risk. These examples constitute one of the main challenges to the received view on the foundations of decision theory offered by Leonard Savage in Savage (1972). Craig Fox and Amos Tversky have, nevertheless, offered an indirect defense of Savage. They provided in Fox and Tversky (1995) an explanation of Ellsberg's two-color problem in terms of a psychological effect: ambiguity aversion. The 'comparative ignorance' hypothesis articulates how this effect works and explains why it is important to an understanding of the typical pattern of responses associated with Ellsberg's two-color problem. In the first part of this article we challenge Fox and Tversky's explanation. We present first an experiment that extends Ellsberg's two-color problem where certain predictions of the comparative ignorance hypothesis are not confirmed. In addition the hypothesis seems unable to explain how the subjects resolve trade-offs between security and expected pay-off when vagueness is present. Ellsberg offered an explanation of the typical behavior elicited by his examples in terms of these trade-offs and in section three we offer a model of Ellsberg's trade-offs. The model takes seriously the role of imprecise A preliminary version of the experiment presented in Sect. 2 was first presented in the 4th International Symposium on Imprecise Probabilities and Their Applications, Pittsburgh, Pennsylvania, 2005. The experiment for the three color problem and the analysis of Ellsberg's trade-offs was discussed in a paper presented in the Workshop on Rationality and Knowledge organized as part of European Summer School on Logic, Language and Information: ESSLLI 2006. 123 38 Synthese (2010) 172:37-55 probabilities in explaining Ellsberg's phenomenon. The so-called three-color problem was also considered in Fox and Tversky (1995). We argue that Fox and Tversky's analysis of this case breaks a symmetry with their analysis of the two-color problem. We propose a unified treatment of both problems and we present a experiment that confirms our hypothesis.

appropriate to make some general comments on the conference theme since it might not be clear what sorts of philosophically interesting relations exist between epistemology and economics. After all, epistemology is often identified with attempts to analyze terms like knows and believes, while much of classical economics (e.g., consumer theory, the theory of the firm) can be understood as being about rational agents and the various equilibria that could result from their interactions. The following is a brief survey of just some of the philosophically interesting relations between epistemology and economics:

multiattribute, revealed preference, descriptive, uncertainty, methodology, D12, D81,