De Finetti proposed a thought experiment along something like the following lines: You must set the price of a promise to pay $1 if there was life on Mars 1 billion years ago, and $0 if there was not, and tomorrow the answer will be revealed. You know that your opponent will be able to choose either to buy such a promise from you after you at the price you have set, or require you to buy such a promise from your opponent, still at the same price. In other words: you set the odds, but your opponent decides which side of the bet will be yours. The price you set is the "operational subjective probability" that you assign to the proposition on which you are betting.
De Finetti is also noted for de Finetti's theorem on exchangeable sequences of random variables. It was de Finetti who introduced this concept of exchangeability.
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