JAK wrote:Godel's Incompleteness Theorem
Zillion's Philosophy Pages
First let me try to state in clear terms exactly what he proved, since some of us may have sort of a fuzzy idea of his proof, or have heard it from someone with a fuzzy idea of the proof..
The proof begins with Godel defining a simple symbolic system. He has the concept of a variables, the concept of a statement, and the format of a proof as a series of statements, reducing the formula that is being proven back to a postulate by legal manipulations. Godel only need define a system complex enough to do arithmetic for his proof to hold.
Godel then points out that the following statement is a part of the system: a statement P which states "there is no proof of P". If P is true, there is no proof of it. If P is false, there is a proof that P is true, which is a contradiction. Therefore it cannot be determined within the system whether P is true.
As I see it, this is essentially the "Liar's Paradox" generalized for all symbolic systems. For those of you unfamiliar with that phrase, I mean the standard "riddle" of a man walking up to you and saying "I am lying". The same paradox emerges. This is exactly what we should expect, since language itself is a symbolic system.
Godel's proof is designed to emphasize that the statement P is *necessarily* a part of the system, not something arbitrary that someone dreamed up. Godel actually numbers all possible proofs and statements in the system by listing them lexigraphically. After showing the existence of that first "Godel" statement, Godel goes on to prove that there are an infinite number of Godel statements in the system, and that even if these were enumerated very carefully and added to the postulates of the system, more Godel statements would arise. This goes on infinitely, showing that there is no way to get around Godel-format statements: all symbolic systems will contain them.
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Also recognize that Godel is one of many in Symbolic Logic: Theory and Practices
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JAK
True to my prediction, JAK rushes onto Google and cuts and pastes a synopsis of Godel's theorom. He then thinks, given he's studied, or shall we say, Googled formal logic for two days now, he's going to show us something new, that Godel isn't the only logician who's walked the earth. No, he's not, but his contribution is the single most important contribution to mathematical logic. He's an extremely unlikely candidate for offering a proof that is circular.
Funny though in all this, JAK found a missed word from me in one of my quotes as a way around owning up to the fact that he doesn't know what a tautology is.