Thursday, October 01, 2015

Measuring prior probability

Robin LePoidevin has written sympathetically about atheism and agnosticism. But a few years ago he made an interesting observation. He begins by stating a stock objection to theism:

The default position in any debate is whichever view is less likely to be true. The more improbable the hypothesis, the greater the need for justification. Theism is intrinsically less likely than atheism, so it stands in greater need of justification.

To which he responds (in part):

We need some means of establishing the likelihood of a hypothesis…perhaps we can measure the prior probability of a hypothesis by how much it rules out. The more it rules out, the lower the prior probability. The less it rules out, the greater the prior probability. Robin LePoidevin, Agnosticism: A Very Short Introduction (OUP, 2010), 49-50.

But assuming that's a sound principle, doesn't physicalism rule out much more than Christian theism? It precludes abstract objects (i.e. numbers). It precludes immaterial minds. Indeed, some physicalists deny consciousness altogether. Likewise, the denial of miracles is a universal negative. 

But by LePoidevin's logic, that means Christian theism has a higher prior probability than physicalism and/or atheism. And that's even before we add all the specific evidence for Christian theism.

1 comment:

  1. Your last paragraph doesn't follow, because Christian theism and physicalism do not form what is known as a partition. So a lower prior probability for one doesn't automatically result in a gain in prior probability for the other. Both could be equally low while some other hypothesis or hypotheses fill up the remainder of the probability space.

    However, I think that physicalism has (literally) a probability of zero for roughly Cartesian reasons--that is to say, I can know with certainty that I exist and that I am not a physical thing. So theism, as long as it has a probability greater than zero, is in a better place than physicalism.

    However, that doesn't mean that it has a higher _prior_ probability (where that is clearly defined in relation to some background evidence) than some sort of non-physicalist atheism. Suppose one is a mind-body dualist but just doesn't believe that the non-physical world includes God, for example. Throw in ethical Platonism to give a grounding for absolute moral truths, and one has at least a workable form of non-theism with respect to some fairly minimal level of human experience.

    Suppose that, taken _prior_ to _all_ arguments of natural theology, all specific evidence from miracles, all of it, that form of atheism has a higher probability than Christian theism. It doesn't really matter. So what? Nobody sits around arguing about the _prior_ probability that Barack Obama exists! That is to say, the probability _aside from_ all of the actual evidence for his existence. *Of course* the existence of a person with exactly that name and those characteristics, who becomes the first black President of the U.S., has a low probability *prior* to any specific evidence on the matter. And so what? What we should be interested in is the probability when evidence is taken into account. And that is overwhelmingly high for Barack Obama's existence and, a good apologist will argue, for the existence of the Christian God as well.