The supporting argument's P3 smuggles in a contentious Bayesian assumption: that rationality requires beliefs representable as single probability measures rather than sets of measures.

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Bayesian assumption(as used in epistemology (the study of knowledge)): A way of thinking about probability based on updating beliefs as you get new information — named after Thomas Bayes, an 18th-century mathematician. Here it means assuming beliefs should work like probabilities.
P3(as referenced in formal logic or philosophical argumentation): A specific rule or principle (probably the third one in a list) being discussed in the larger argument.
Probability measure(what the Mentaculus fails to provide for the Future Hypothesis): A mathematical tool that assigns probabilities to different possible outcomes in a consistent way, so they add up correctly and follow the rules of probability.
Sets of measures(as used in formal epistemology): Multiple different probability assignments for the same belief, rather than just one single number — like saying 'the probability is somewhere between 60% and 80%' instead of exactly 70%.

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