By defining rationality as best way of thinking which could be presented in finite set of rules, we hope that this definition would converge into a one and only finite object, the best set of rules, and in this case we would be able to say that rationality actually exist.
I think the average LW users would consider that to be Bayesian probability, which indeed have been used as the basis for an idealized AI (called AIXI).
I think the average LW users would consider that to be Bayesian probability, which indeed have been used as the basis for an idealized AI (called AIXI).