Skeptical scenarios (brains in vats, Descartes' demon, sophisticated simulation) challenge closure by seemingly showing we can know ordinary propositions (I have hands) without knowing we're not in skeptical scenarios (I'm not a brain in a vat), even though the former entails the latter. This forces epistemology to choose between accepting skepticism, rejecting closure, or distinguishing levels of knowledge. The tension illustrates a fundamental problem in the foundations of knowledge.
Examine skeptical hypotheses (brain in a vat, evil demon, simulation) and identify the intuition driving closure: if you don't know you're not in the skeptical scenario, how can you know ordinary facts? Test proposed resolutions.
From your study of external world skepticism, you know the basic skeptical problem: our sensory evidence is compatible with radically different underlying realities. From your study of epistemic closure, you know the principle: if you know P, and you know that P entails Q, then you know Q (or are in a position to know Q). Skeptical scenarios with closure arguments combine these into one of the sharpest challenges in epistemology. The argument goes quickly from ordinary common sense to apparent disaster.
Here is the core inference. You believe you have hands — you can see them, feel them, you've used them all your life. Call this belief H. Now consider a skeptical scenario: a brain in a vat (BIV), connected to a computer that produces perfectly coherent simulated experiences of having hands and a body. If you were a BIV, your experiences would be indistinguishable from your actual experiences. Now, H (I have hands) entails not-BIV (I am not a brain in a vat), because if you're a brain in a vat, you don't actually have hands. By closure, if you know H, you must know not-BIV. But can you know not-BIV? You have no experience or evidence that discriminates between the real-hands scenario and the BIV scenario. It seems you can't rule out BIV. Therefore, by modus tollens running the closure argument backwards, perhaps you don't know H either. The ordinary knowledge you took for granted is apparently undermined.
Three major response strategies have been developed. The first is accepting skepticism: bite the bullet and acknowledge that we don't know ordinary propositions in the strict philosophical sense. This is intellectually honest but practically bizarre — we can't operate as though we don't know we have hands. The second is rejecting closure: some philosophers (notably Fred Dretske and Robert Nozick) argue that knowledge does not transmit through entailment in this way. On a tracking account, you know H because your belief in H tracks the truth — you would not believe you have hands if you didn't. But you don't need your belief in not-BIV to track its truth. Knowledge can be compartmentalized. The third strategy is contextualism: what counts as "knowing" depends on the context of inquiry. In an ordinary conversation, saying "I know I have hands" is true; in a skeptical philosophical seminar where BIV scenarios are explicitly raised, the standards for "knowing" are elevated and the claim might be false. Knowledge attributions are context-sensitive, not absolute.
Your soft prerequisites in possible worlds semantics are useful here. Tracking accounts can be expressed in modal terms: you know P if there is no nearby possible world in which you falsely believe P. BIV worlds are not "nearby" in the relevant sense — they are highly unusual alternative scenarios, not close variations on your actual situation. This is why the tracking theorist can say: I know I have hands (no close world where I'm wrong about this), even though I don't know I'm not a BIV (the BIV world, though distant, is indistinguishable from this one and I couldn't detect the difference). The modal framework makes precise what "relevant alternatives" means and which possibilities must be eliminated for knowledge.
The philosophical importance of this topic extends beyond academic puzzle-solving. How you resolve the tension between closure and skeptical scenarios reveals your deeper commitments about what knowledge is for. If knowledge requires ruling out every logically possible alternative, it becomes unattainable. If it requires ruling out only practically relevant alternatives, it is attainable but the notion of "relevance" needs an account. The skeptical scenarios argument is a stress test: a theory of knowledge that cannot explain why ordinary people genuinely know they have hands, while also explaining why we cannot empirically rule out BIV scenarios, is in trouble. The solutions (denying closure, contextualizing knowledge, invoking possible worlds) are also the leading positive theories of knowledge in contemporary epistemology.
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