Scepticism/Closure Flashcards
What is the deductive closure principle?
If s knows p, and s correctly deduces q from p, then s knows q.
How can the sceptic appeal to the closure principle?
We cannot know sceptical alternatives to be false.
q=I’m a brain-in-a-vat.
If s knows p and correctly deduces not-q from p then s knows not-q. Since we don’t know sceptical alternatives to be false, we fail to know p.
Sceptical alternatives = (s knows q on basis of e iff e entails q) proposition incompatible with q but compatible with e.
How might we respond to the sceptic’s deductive closure principle?
- Deny closure principle (relevant alternatives fallibilism).
- Accept closure principle, use it against sceptic (modus ponens fallibilism).
What is modus ponens fallibilism?
Start with claim we do know a lot about the world. Follows that we know sceptical alternatives to be false. I know looking at paper (p) looking at paper precludes being brain-in-a-vat (q) therefore I know I’m not a brain-in-a-vat (not-q).
How might one respond to modus ponens fallibilism?
Obj: is reasoning legitimate? Sceptical alternatives arise because we don’t know they’re false.
Rep: not-q inferred from p. Can’t use evidence, doesn’t follow I have no reason to believe p.
Obj: r=car parked in front of store; t=car hasn’t been towed away. Insufficient to infer t from r.
What is relevant alternatives fallibilism?
Deny sceptical alternatives are relevant: ‘alternative to p relevant in case we have to know falsity of q in order to know p’.
How? Indirect: construct theory of knowledge that has failure of closure as consequence. Nozick’s subjunctive conditionals - ‘if q were false, I wouldn’t believe q’. E.g., I wouldn’t believe I’m seeing paper in front of me if there wasn’t paper in front of me.
How might one respond to modus ponens fallibilism?
Obj: intuitions no more compelling than those that support closure.
Rep: Moorean rationality of intuitions. More rational to accept intuitions in favour of counter examples even if we can’t show sceptical alternatives to be false.
C.obj: not philosophically rigorous.
C.rep: everyday knowledge attribution v. compelling.
