Epistemology
External World Skepticism
How do we know there's an external world? — radical philosophical doubt
External world skepticism asks: how do we know there's a world beyond our minds? Can we trust senses? Modern formulations: brain in a vat (BIV), evil demon (Descartes), simulation hypothesis. The skeptic argues: experience could be identical whether external world exists or not. Therefore: we lack knowledge of external world. Major responses: (1) Refute by showing conceptual incoherence (Putnam). (2) Closure principle: I know I have hands → external world exists (Moore, "Here is one hand"). (3) Pragmatic: doesn't matter for life. (4) Bayesian: simulation/BIV unlikely.
- QuestionHow can we know external world exists?
- Descartes' demonEvil being deceiving senses
- Brain in vatModern version
- Simulation hypothesisComputer simulation we can't detect
- Major responsesPutnam, Moore, pragmatic, Bayesian
- Famous responseG.E. Moore, "Here is one hand"
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Why external world skepticism matters
- Epistemology. Limits of knowledge.
- Philosophy of mind. Reality vs experience.
- Simulation hypothesis. Modern relevance.
- Cognitive science. Reliability of senses.
- Religion. Faith vs evidence.
- Critical thinking. Examining basic assumptions.
- Education. Foundation of philosophy.
Common misconceptions
- Skeptic believes simulation. Argues we can't rule out.
- Settled by science. Different question; not empirical.
- Just intellectual game. Substantial epistemological issue.
- Refuted easily. Active debate; multiple proposed responses.
- External world denial. Often agnosticism, not denial.
- Common sense settles it. Common sense itself examined.
Frequently asked questions
What's external world skepticism?
Radical form of skepticism: we don't (or can't) know external physical world exists. Our experience of the world: consistent with no external world (just mental states). Proposed scenarios. (1) Descartes' evil demon: powerful being deceiving senses about reality. (2) Brain in a vat: stimulating brain to produce illusion of world. (3) Simulation hypothesis: we live in computer simulation. Each: experiences would be identical to real-world case. Therefore: no way to distinguish.
What's the brain-in-vat scenario?
Modern formulation. Imagine: your brain is in a vat, hooked to wires, getting electrical signals identical to real-world stimulation. Experience: indistinguishable from real life. Could you tell? Apparently not. Suggests: you don't know external world. Hilary Putnam (1981) gave influential discussion. Note: doesn't claim we ARE BIVs; argues we can't rule it out.
What's the simulation hypothesis?
Modern variant. We live in computer simulation. Could be running on ancestor simulation — future civilization simulating past. Bostrom (2003): probabilistic argument we likely live in simulation. Implications: physical world is information; simulators control reality. Not just sci-fi: serious philosophical discussion. Updates ancient skeptical arguments.
What's Moore's response?
G.E. Moore (1939): "Here is one hand. Here is another. So there are at least two hands. So there is an external world." Refutes skepticism by appeal to common sense. Skeptic argument: less certain than premise (I have hands). If skeptical conclusion absurd, premise must be wrong. Critics: assumes what's at issue. But: pragmatic appeal — common sense as starting point.
What's Putnam's response?
Hilary Putnam (1981). Argued BIV scenario incoherent. Words refer to real things in real world; if we're in a vat, "brain in vat" can't refer to real brain in vat (since we wouldn't have the right causal connection). So: "we are brains in vats" can't be true. Semantic externalism solves the problem. Controversial — some find argument convincing, others dismiss.
How does it relate to Descartes?
Descartes used skepticism methodologically — to find indubitable foundation. Doubted external world; arrived at cogito ergo sum (I think therefore I am) as certain. Then: built outward (God exists, external world exists). Modern skeptics: don't necessarily believe skepticism, but ask: how do we know? Tests epistemology.
What do philosophers conclude?
Various positions. (1) Skepticism is true: we can't know external world. (2) Skepticism is false: have arguments to refute (Moore, Putnam, etc.). (3) Skepticism is unanswerable but irrelevant: we can't refute it but can't act on it. (4) Reformulate epistemology: knowledge isn't certainty (fallibilism). Most philosophers: not full skeptics; debate continues.