Depth 2 · 34 s
A brain in a jar
Could your whole world be made up for a brain kept in a jar? The sceptic's argument in three steps, Moore's "here is one hand", and the answer that knowledge never needed certainty.
About this video
Imagine a scientist far in the future. She keeps a brain alive in a jar and connects it to a computer, which sends it exactly the signals it would get from eyes, ears and skin. The brain "sees" a kitchen, "feels" a cup of tea, "hears" a friend laughing - but none of it is there. Everything you are seeing right now would feel just the same if you were that brain. Can you prove you aren't?
The dream question asked how you know you aren't dreaming. The brain in a jar makes it sharper: a dream ends and you wake up, but the brain in the jar never wakes. Hilary Putnam made the example famous in 1981 (a brain in a vat); Descartes had a similar one in 1641, an evil demon who fools you about everything.
A sceptic turns the example into an argument with three steps. If I know I have hands, then I know I am not a brain in a jar. But I can't know I am not a brain in a jar. So I don't know I have hands. The argument looks valid: if the first two lines are true, the last must be too. So to say you know you have hands, you must say which of the first two lines is false.
In 1939 G. E. Moore held up his hand and said "Here is one hand", then the other. He was far more sure he had hands than of any step in a clever argument, so he turned the argument around: I do know I have hands, so I know I am not a brain in a jar. Same first line, opposite ending. A second answer says knowledge never needed certainty - only good reasons, a true belief, and a way of believing that wouldn't easily lead you wrong. Neither answer satisfies everyone, and the debate goes on. This video goes with the A Brain in a Jar lesson of Knowledge Level 2 on Knaph.
This video shows depth 2 of Scepticism.
Learn ScepticismTranscript
- 0:00A brain in a jar, fed a whole world by a computer.
- 0:05Everything you see would feel the same. Can you prove you aren't?
- 0:11The sceptic. Can't rule out the jar? Then you don't know your hands.
- 0:17Moore turned it round. Here is one hand, so I'm not in a jar.
- 0:23Another answer. Knowledge never needed certainty, only good reasons.
- 0:29Which are you surer of?