ABSTRACT
This chapter ranges from Thucydides to Oskar Morgenstern. It indicates that conjectures have always been needed in decision-making, and outlines the position assigned to them in the modern theory of games and decisions. A mistaken conjecture can be disastrous. Through a mistaken conjecture Hitler was made chancellor of the Reich. One of the great merits of the modern theory of "games and decisions" is that it requires actors to state decision-problems clearly. If the probabilities of the states of the world are known, the criterion adopted is the "expectation". To find the expectation of an action, multiply the values resulting from that action in different states by the corresponding probabilities of obtaining the values, then add the products together, and divide by the number of states. Logical tools can always be used in order to set up an arbitrary classification of states.
