Ideal rationality and hand waving

Australasian Journal of Philosophy 68 (2):147 – 156 (1990)
In discussions surrounding epistemology and rationality, it is often useful to assume an agent is rational or ideally rational. Often, this ideal rationality assumption is spelled out along the following lines: 1. The agent believes everything about a situation which the evidence entitles her to believe and nothing which it does not. 2. The agent believes all the logical consequences of any of her beliefs. 3. The agent knows her own mind: if she believes P, she believes that she believes P; and if she doesn't believe P, she believes that she doesn't believe P. 4. The agent believes nothing of the form 'P and it is not the case that P.' 5. If an agent's background belief-set satisfies 1-4 and if rationality requires the agent to add P to her belief-set, then the resulting belief-set will also satisfy 1-4. While individually plausible, there are cases in which holding on to 1-5 generates paradox. Some resolve such paradoxical cases by granting 1-5 but arguing while ideally rational agents can exist, they can't possibly ever find themselves in such a situation: such case descriptions are epistemically incoherent. Others allow that rational agents can coherently find themselves in such odd circumstances, and argue that it's more reasonable to weaken our concept of ideal rationality and give up premise (2) above. However, this strategy has also been rejected. My aim in this paper is to defend the utility of positing an ideally rational agent in such paradoxical circumstances. I argue in such cases we should give up (2), in particular the assumption that (necessarily) if an ideally rational agent believes both P and the conditional, if P then Q, then she believes Q.. What's important is to hold on to the goal of positing ideal rationality: to maximize the amount of true or probably true information a thinker can justifiably believe in a given circumstance. Normally that will mean holding on to (2), but these unusual paradoxical cases are best handled by giving up (2).
Keywords rationality  epistemic rationality  ideally rational agent  rational agent  closure  deductive closure  belief  rational belief
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DOI 10.1080/00048409012344171
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David Christensen (2010). Higher-Order Evidence. Philosophy and Phenomenological Research 81 (1):185-215.
Earl Conee (1994). Against an Epistemic Dilemma. Australasian Journal of Philosophy 72 (4):475 – 481.

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