  The Ellsberg and Machina paradoxes reveal that expected utility theory is
problematical when real subjects take decisions under uncertainty. Suitable
generalizations of expected utility exist which attempt to solve the Ellsberg
paradox, but none of them provides a satisfactory solution of the Machina
paradox. In this paper we elaborate a quantum model in Hilbert space describing
the Ellsberg situation and also the Machina situation, and show that we can
model the specific aspect of the Machina situation that is unable to be modeled
within the existing generalizations of expected utility.
