On the Definition of States and the Choice Set in the Theory of Decision Making under Uncertainty Edi Karni ∗ September 14. 2015 Abstract This paper discusses the definition of the state space and corre- sponding choice sets that figure in the theory of decision making under uncertainty. It elucidates an approach that overcomes some concep- tual difficulties with the standard models and accommodates a proce- dure for expanding the state space in the wake of growing awareness. . . Keywords: States of nature, decision making under uncertainty, resolution of uncertainty, choice set. . . JEL classification numbers: D81 ∗ Department of Economics, Johns Hopkins University and Warwick Business School, University of Warwick. Part of this work was conducted during my visits to EIEF, Rome and the Berglas School of Economics at Tel Aviv University. I am grateful to both in- stitutions for their support and hospitality. I am grateful to Itzhak Gilboa, Zvi Safra, Peter Hammond, Jean Baccelli, and Marie-Louise Vierø for their useful comments and suggestions. 1