The secret technical codeword that cognitive scientists use to mean "rational". Bayesian probability theory is the math of epistemic rationality, Bayesian decision theory is the math of instrumental rationality. Right up there with cognitive bias as an absolutely fundamental concept on Less Wrong.
Classical statistics is a bucket of assorted methods; different "methods" may give different answers for whether, e.g., an experimental result is "statistically significant". In contrast, as the famous Bayesian E. T. Jaynes emphasized, probability theory is math and its results are theorems, every theorem consistent with every other theorem; you cannot get two different results by doing the derivation two different ways.
So is the project of rationality solved? Indeed not. First, probability theory and decision theory are often too computationally expensive to run in practice - it wouldn't take a galaxy-sized computer, so much as an unphysical computer (much larger than the known universe). And second, it's not always clear how the math applies - even in theory, let alone the practice....