A review of Swinburne's BookNote:
The most problematical assertion in Chapter 1 is the following: ‘It is a further fundamental epistemological principle additional to the principle that other things being equal we should trust our memories, that we should believe what others tell us that they have done or perceived—in the absence of counter-evidence. I call this the principle of testimony. It must be extended so as to require us to believe that—in the absence of counter-evidence—when someone tell us that so-and-so is the case…they have perceived or received testimony from others that it is the case. Without this principle we would have very little knowledge of the world.’ (p. 13)
which implies that Swinburne' calculations have to take everyone at their word.
Also note this:
Assigning probabilities to the factors of equation 4′ is done by building up from other factors: ‘Let us represent by t theism, the claim that there is a God of the traditional kind. P(t/k) is the probability that there is such a God on the evidence of natural theology. I suggested in Chapter 1 that we give this the modest value 1∕2.’ (p. 211) Swinburne backs up this value only in the last paragraph of Chapter 1: ‘This evidence, the evidence of natural theology, provides general background evidence crucially relevant to our topic. I have argued elsewhere the case for this evidence giving substantial probability to the existence of God. (See esp. my The Existence of God and the shorter Is there a God? (Oxford University Press, 1996)). I cannot, for reasons of space, argue that case again here. But to get my argument going here, I will make only the moderate assumption that the evidence…makes it as probable as not that there is a God…’ (p. 30) I’ll return to more in Chapter 1, Principles for Weighing Evidence, after another illustration of assigning probabilities.
So, the first "one in two chance" is Swinburne's subjective claim that, given what he sees as the evidence from what we see in the world, then it is a 0.5 probability that it is a theistic god. Also, we see that Swinburne's equation is
P(h/e&k) = P(e/h&k)P(h/k)∕P(e/k)
and the meanings he assigns to the letters are:
Specifying the factors in equation 4′, Swinburne states, ‘Let k now be…the evidence of natural theology (including the sinning and suffering of humans). Let e be the detailed historical evidence, consisting of a conjunction of three pieces of evidence (e1 &e2 &e3 ). e1 is the evidence of the life of Jesus set out in Part II. e2 is the detailed historical evidence relating to the Resurrection set out in Part III. e3 is the evidence (summarized in Chapter 3) that neither the prior nor the posterior requirements for God being incarnate were satisfied in any prophet in human history in any way comparable with the way in which they were satisfied in Jesus.’ (p. 210) ‘Let h1 be the hypothesis that God became incarnate in Jesus, and h2 the hypothesis that Jesus rose from the dead. h is the conjunction (h1 &h2 ). Now at the end of the day this book is interested in P(h∕e&k)—the probability that Jesus was God Incarnate who rose from the dead (h), on the evidence both of natural theology (k) and of the detailed history of Jesus and of other human prophets (e).’ (p. 211)
So, Swinburne is saying we have historical evidence, that we should trust unless there's a good reason not to. I'd call that naive and gullible, myself, unless he factors in the chances of someone making up stories for the sake of gaining power. It's this that gets him the 0.97 figure - he divides another probability by "the chance that we get the 'historical evidence', just given the state of our world". He thinks there's only 1 in 1000 probability that people would make shit up to found a religion. :eyes: I'd be interested if he has applied his probabilities to other, mutually exclusive religions, and if he think very unlikely that someone would make stuff up in them either.