In decision theory, the sure-thing principle states that a decision maker who decided they would take a certain action in the case that event E has occurred, as well as in the case that the negation of E has occurred, should also take that same action if they know nothing about E.
The principle was coined by L.J. Savage: [1]
A businessman contemplates buying a certain piece of property. He considers the outcome of the next presidential election relevant. So, to clarify the matter to himself, he asks whether he would buy if he knew that the Democratic candidate were going to win, and decides that he would. Similarly, he considers whether he would buy if he knew that the Republican candidate were going to win, and again finds that he would. Seeing that he would buy in either event, he decides that he should buy, even though he does not know which event obtains, or will obtain, as we would ordinarily say. It is all too seldom that a decision can be arrived at on the basis of this principle, but except possibly for the assumption of simple ordering, I know of no other extralogical principle governing decisions that finds such ready acceptance.
— p. 21
Savage formulated the principle as a dominance principle, but it can also be framed probabilistically. [2] Richard Jeffrey [2] and later Judea Pearl [3] showed that Savage's principle is only valid when the probability of the event considered (e.g., the winner of the election) is unaffected by the action (buying the property). Under such conditions, the sure-thing principle is a theorem in the do-calculus [3] (see Bayes networks). Blyth constructed a counterexample to the sure-thing principle using sequential sampling in the context of Simpson's paradox, [4] but this example violates the required action-independence provision. [5]
In the above cited paragraph, Savage illustrated the principle in terms of knowledge. However the formal definition of the principle, known as P2, does not involve knowledge because, in Savage's words, "it would introduce new undefined technical terms referring to knowledge and possibility that would render it mathematically useless without still more postulates governing these terms." Samet [6] provided a formal definition of the principle in terms of knowledge and showed that the impossibility to agree to disagree is a generalization of the sure-thing principle. It is similarly targeted by the Ellsberg and Allais paradoxes, in which actual people's choices seem to violate this principle. [2]
In logic, the law of non-contradiction (LNC) states that contradictory propositions cannot both be true in the same sense at the same time, e. g. the two propositions "p is the case" and "p is not the case" are mutually exclusive. Formally, this is expressed as the tautology ¬(p ∧ ¬p). The law is not to be confused with the law of excluded middle which states that at least one, "p is the case" or "p is not the case", holds.
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Meno is a Socratic dialogue written by Plato. Meno begins the dialogue by asking Socrates whether virtue is taught, acquired by practice, or comes by nature. In order to determine whether virtue is teachable or not, Socrates tells Meno that they first need to determine what virtue is. When the characters speak of virtue, or rather arete, they refer to virtue in general, rather than particular virtues, such as justice or temperance. The first part of the work showcases Socratic dialectical style; Meno, unable to adequately define virtue, is reduced to confusion or aporia. Socrates suggests that they seek an adequate definition for virtue together. In response, Meno suggests that it is impossible to seek what one does not know, because one will be unable to determine whether one has found it.
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