Arcesilaus's challenge, preserved in Sextus Empiricus, is that indiscernibility of false impressions collapses the cognitive impression into a mere externalist definition with no action-guiding force.

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Action-guiding force(in ethics): The ability of a principle or rule to actually tell us what we should do in real situations.
Arcesilaus(the main person discussed in this statement): An ancient Greek philosopher (around 315-240 BCE) who led the Skeptical Academy and argued that we cannot know anything for certain, but we can still make reasonable judgments to guide our actions.
Externalist definition(as what the cognitive impression allegedly collapses into): A way of defining knowledge based only on outside, objective facts rather than on what's happening inside your mind or how confident you feel.
Indiscernibility of false impressions(as the core problem Arcesilaus raises): The problem that fake or wrong impressions can look completely identical to real or correct ones—so you can't tell them apart just by looking at them.

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