# Incompleteness theorem

1. Apr 6, 2006

### heman

The proof of Gödel's Incompleteness Theorem is so simple, and so sneaky, that it is almost embarassing to relate. His basic procedure is as follows:

1. Someone introduces Gödel to a UTM, a machine that is supposed to be a Universal Truth Machine, capable of correctly answering any question at all.
2. Gödel asks for the program and the circuit design of the UTM. The program may be complicated, but it can only be finitely long. Call the program P(UTM) for Program of the Universal Truth Machine.
3. Smiling a little, Gödel writes out the following sentence: "The machine constructed on the basis of the program P(UTM) will never say that this sentence is true." Call this sentence G for Gödel. Note that G is equivalent to: "UTM will never say G is true."
4. Now Gödel laughs his high laugh and asks UTM whether G is true or not.
5. If UTM says G is true, then "UTM will never say G is true" is false. If "UTM will never say G is true" is false, then G is false (since G = "UTM will never say G is true"). So if UTM says G is true, then G is in fact false, and UTM has made a false statement. So UTM will never say that G is true, since UTM makes only true statements.
6. We have established that UTM will never say G is true. So "UTM will never say G is true" is in fact a true statement. So G is true (since G = "UTM will never say G is true").
7. "I know a truth that UTM can never utter," Gödel says. "I know that G is true. UTM is not truly universal."

Think about it - it grows on you ...

i really dont get it...i get lost understanding it..

2. Apr 8, 2006

### topsquark

I see nothing in the statement that says that UTM can't opt to say nothing...

-Dan

3. Apr 8, 2006

### Hurkyl

Staff Emeritus
Then it wouldn't be a "Universal Truth Machine, capable of correctly answering any question at all".

It would instead be a partial truth machine that answers some questions correctly, and doesn't answer other the other questions.

4. Apr 8, 2006

### gravenewworld

Think about a liar's type paradox- If I write "The following sentence is false. The preceeding sentence is true" Now is that system of sentences consistent? Can you determine truth or falseness from them? No-it is a paradox. This is "essentially" what Godel did, although a Godel statement G is a paradox due to (dis)provability rather than truth/falseness.