By ‘Bayesian’ philosophy of science I mean the position that (1) the objective of science is, or should be, to increase our ‘credence’ for true theories, and that (2) the credences held by a rational thinker obey the probability calculus. However, if *T* is an explanatory theory (e.g. ‘the sun is powered by nuclear fusion’), then its negation *~T* (‘the sun is not powered by nuclear fusion’) is not an explanation at all. Therefore, suppose (implausibly, for the sake of argument) that one could quantify ‘the property that science strives to maximise’. If *T* had an amount *q* of that, then *~T* would have none at all, not 1-*q* as the probability calculus would require if *q* were a probability.

Also, the conjunction (*T₁ *&** ***T₂*) of two mutually inconsistent explanatory theories *T₁ *and** ***T₂* (such as quantum theory and relativity) is provably false, and therefore has zero probability. Yet it embodies some understanding of the world and is definitely better than nothing.

Furthermore if we expect, with Popper, that all our best theories of fundamental physics are going to be superseded eventually, and we therefore believe their negations, it is still those false theories, not their true negations, that constitute all our deepest knowledge of physics.

What science really seeks to ‘maximise’ (or rather, create) is explanatory power.