Ya I suppose my point is that such assignments are inherently subjective, and derived from limited data (except in some limited cases, e.g. disease <i>screening</i>, though not disease testing in general).<p>Consider the theory that there is an unidentified teapot floating around the moon. What prior would you assign that? 1x10^-20? What if I then told you that a teapot was on board the space shuttle during the first moon landing? Maybe then the exponent rises to -10. What if then I further told you that when it came back, that teapot was unaccounted for? Now all of the sudden what seemed wildly improbable actually seems remotely possible. Not because anything changed in the world, mind you, but because your knowledge of the world became more complete.<p>Particularly in the case of a scientific theory, it seems almost completely impossible to adequately estimate the probability of its truth prior to testing. For instance, if I were to propose that conservation of energy is incorrect, most people would assign that an extraordinarily low prior. But would they really be justified in so doing? Is there <i>really</i> evidence that conservation of energy is a fundamental property of the universe, or does there just happen to be a lot of stuff that we&#x27;ve observed that comports with it? I suppose i&#x27;m essentially making Plato&#x27;s cave&#x27;s argument here.