PART 2: GO ON, TAKE A CHANCE How much evidence do we need?

William James vs. William Clifford Smackdown! The battle of bearded men named William

James and Clifford agree that Our goal as rational thinkers should be to gain truth and avoid error. If nearly all the evidence supports some claim P and there is practically no decent evidence against P, then the rational thing to do is believe P. P is mostly probably true.

What if the evidence is inconclusive? Three possible things to do: 1. Withhold belief about P. Just refuse to either believe or deny P until more evidence comes in that clearly settles the matter about P and you can make a more informed decision. 2. Go ahead and believe P anyway. 3. Go ahead and deny P anyway.

James vs. Clifford on inconclusive evidence Clifford: choose option 1. Withhold belief until you have sufficient evidence one way or another. James: setting the evidence bar high, like Clifford does, is great for avoiding errors. You’re not very likely to be wrong. But it is lousy for gaining truths. If you don’t believe very many things, then you don’t believe very many true things either.

Why prefer avoid error over gain truth? Clifford adopts Risk averse principle: better off to miss out on some truths rather than add more errors. James says it is equally rational to adopt Risk positive principle: better off to add more errors rather than miss out on some truths.

An analogue of risk averse and risk positive principles Risk averse principle (pleasure/pain version): better off to miss out on some pleasures rather than add more pain Risk positive principle (pleasure/pain version): better off to add more pain rather than miss out on some pleasures Which seems better to you?

Another analogue of the two principles Risk averse principle (guilt/innocence version): better off to let some guilty go free rather than convict more innocent people Risk positive principle (guilt/innocence version): better off to convict more innocent people rather than let any guilty people go free Which seems better to you?

The basic conflict between James and Clifford Clifford: the risk averse principle alone is rational James: the risk averse principle and the risk positive principle are equally rational

Implications of James’s view Suppose there is equal evidence for and against some claim P This means there is evidence for P and evidence for not-P By the risk positive principle it is rational to believe P. But it is also rational to believe not-P! It is a problem that belief and disbelief are equally reasonable?

A problem for James If there is zero evidence for a claim, James agrees with Clifford that we should withhold belief. If the evidence for a claim is exactly balanced by the evidence that the claim is false, why don’t they cancel each other out? Why isn’t it equivalent to having no evidence? Therefore a case of mixed evidence is really a case of no evidence. Clifford is right after all!