In his Philosophical Theories of Probability, Donald Gillies proposes what he calls an intersubjective theory of probability

. A better name for it would be group-strategy model of probability

.

Subjectivists such as Bruno di Finetti ask the reader to consider the following sort of game:

- Some potential event is identified.
- Our hero must choose a real number (negative or positive)
`q`, a *betting quotient*. - The nemesis, who is rational, must choose a stake
`S`, which is a positive or negative sum of money or zero. - Our hero must, under any circumstance, pay the nemesis
`q`·`S`. (If the product `q`·`S` is negative, then this amounts to the nemesis paying money to our hero.) - If the identified event occurs, then the nemesis must pay our hero
`S` (which, if `S` is negative, then amounts to taking money from our hero). If it does not occur, then our hero gets nothing.

Di Finetti argues that a rational betting quotient will capture a rational

*degree of personal belief*, and that a

*probability* is exactly and only a degree of personal belief.

Gillies asks us to consider games of the very same sort, except that the betting quotients must be chosen *jointly* amongst a team of players. Such betting quotients would be at least *examples* of what Gillies calls intersubjective probabilities

. Gillies tells us that these are the probabilities of rational consensus. For example, these are ostensibly the probabilities of *scientific* consensus.

Opponents of subjectivists such as di Finetti have long argued that the sort of game that he proposes fails in one way or another to be formally identical to the *general* problem for the application of personal degrees of belief. Gillies doesn't even *try* to show how the game, if played by a team, is formally identical to the *general* problem of group commitment to propositions. He instead belabors a *different* point, which should already be obvious to *all* of his readers, that *team*work is sometimes in the interest of the individual.

Amongst other things, *scientific method* is about *best approximation of the truth*. There are some genuine, difficult questions about just what makes one approximation *better* than another, but an approximation isn't relevantly better for promoting such things as the social standing as such or material wealth as such of a particular clique. It isn't at all clear who or what, in the formation of genuinely *scientific* consensus, would play a rôle that corresponds to that of the *nemesis* in the betting game.

Karl Popper, who proposed to explain *probabilities* in terms of objective *propensities* (rather than in terms of judgmental orderings or in terms of frequencies), asserted that

Causation is just a special case of propensity: the case of propensity equal to 1, a *determining* demand, or force, for realization.

Gillies joins others in taking him to task for the simple reason that probabilities can be *inverted* — one can talk both about the probability of `A` given `B` and that of `B` given `A`, whereäs presumably if `A` caused `B` then `B` cannot have caused `A`.

Later, for his own propensity theory, Gillies proposes to *define* probability

to apply only to events that display a sort of *independence*. Thus, flips of coins might be described by probabilities, but the value of a random-walk process (where changes are independent but present value is a sum of past changes) would not itself have a probability. None-the-less, while the value of a random walk and similar processes would not themselves have probabilities, they'd still be subject to compositions of probabilities which we would previously have *called* probabilities

.

In other words, Gillies has basically taken the liberty of employing a *foundational notion* of probability, and permitting its *extension*; he chooses not to *call* the extension probability

, but that's *just notation*. Well, Popper had a *foundational* notion of propensity, which is a generalization of causality. He identified this notion with probability, and implicitly extended the notion to include inversions.

Later, Gillies offers dreadful criticism of Keynes. Keynes's judgmental theory of probability implies that every rational person with sufficient intellect and the same information set would ascribe exactly the same probability to a proposition. Gillies asserts

[…] different individuals may come to quite different conclusions even though they have the same background knowledge and expertise in the relevant area, and even though they are all quite rational. A single rational degree of belief on which all rational being should agree seems to be a myth.

So much for the logical interpretation of probability, […].

No two human beings have or *could* have the same information set. (I am reminded of infuriating claims that monozygotic children raised by the same parents have both the same heredity and the same environment.) Gillies writes of the relevant area

, but in the formation of judgments about uncertain matters, we may and as I believe *should* be informed by a very extensive body of knowledge. Awareness that others might dismiss as *irrelevant* can provide support for general relationships. And I don't recall Keynes ever suggesting that there would be real-world cases of two people having the same information set and hence not disagreeing unless one of them were of inferior intellect.

After objecting that the traditional subjective theory doesn't satisfactorily cover all manner of judgmental probability, and claiming that his intersubjective

notion can describe probabilities imputed by groups, Gillies takes another shot at Keynes:

When Keynes propounded his logical theory of probability, he was a member of an elite group of logically minded Cambridge intellectuals (the Apostles). In these circumstances, what he regarded as a single rational degree of belief valid for the whole of humanity may have been no more than the consensus belief of the Apostles. However admirable the Apostles, their consensus beliefs were very far from being shared by the rest of humanity. This became obvious in the 1930s when the Apostles developed a consensus belief in Soviet communism, a belief which was certainly not shared by everyone else.

Note the insinuation that Keynes thought that there were a single rational degree of belief valid for the whole of humanity

, whereäs there is no indication that Keynes felt that everyone did, should, or could have the same information set. Rather than *becoming* obvious to him in the 1930s, it would have *been* evident to Keynes much earlier that many of his own beliefs and those of the other Apostles were at odds with those of most of mankind. Gillies' reference to embrace of Marxism in the '30s by most of the Apostles simply looks like irrelevant, Red-baiting ad hominem to me. One doesn't have to like Keynes (as I don't), Marxism (as I don't) or the Apostles (as I don't) to be appalled by this passage (as I am).