# Godel's incompleteness; result of implication?

1. Jun 8, 2009

### friend

I'm not an expert in mathematical logic by far. But I understand that Godel's incompleteness theorem states that there can be some statements in an arithmatical system that are true but cannot be proven by that system.

But that sounds like an implication. For part of the definition of an implication is that a true conclusion can proceed from a false premise. In other words a true statement can proceed from a false premise, or no true premise... or a true statement without something to prove it is true.

So it sounds like Godel's proof might start out with things being equivalent, but there might be implication introduced somewhere so that the conclusion (about a true statement) might also precede without something to prove it, as well as might procede from something that does prove it. That would only prove that Godel intoduced implication in his proof, right?

2. Jun 8, 2009

### CRGreathouse

I don't understand. Could you rephrase, or maybe give an example of what kind of implication you mean?

3. Jun 8, 2009

### sylas

Yes; any formal system expressive enough to represent arithmetic is either incomplete or inconsistent.

Incomplete means that there are statements S where the system cannot prove either S or (not S).

Inconsistent means that there are statements S where the system can prove both S and (not S).

I don't know what you mean. Godel's theorem is pretty sophisticated. Note that you CAN can provide complete and consistent formal systems for simpler models.

Specifically, consider formal statements involving multiplication and addition of real numbers. There IS a formally complete and consistent logical system for such statements.

But there is no complete and consistent logical system for statements involving multiplication and addition of integers.

So Godel's theorem is not just something weird about "implication".

Certainly not.

Cheers -- sylas

4. Jun 8, 2009

### Phrak

Not quite. This is where you got thrown-off. Some statements cannot be proven nor disproven (in a sufficiently complex system such as algebra, but that can be constructed within the system syntax). This means that they are neither true nor false.

The axioms of the system are insufficient to provide an answer. As the telling goes, one is now free to decide on an additional axiom that will decide the proposition. The new axiom can support the undecidable proposition or deny it.

With the added axiom, the system of axioms is somewhat more complex than the original. Are there now additional propositions that have no truth value as provided by the expanded axiom set?

Last edited: Jun 8, 2009
5. Jun 9, 2009

### Preno

Not really, you may be conflating truth and formal provability within a particular system of first-order logic. The Godel sentence is true because of the way it is constructed.

eta: unless, of course, you view PA as a game of symbols rather than at attempt at axiomatizing natural numbers - in which case you may be right, but then Godel's theorem becomes an uninteresting combinatorial result with no philosophical/metamathematical implications whatsoever.

6. Jun 9, 2009

### HallsofIvy

Staff Emeritus
It is trivially true that "true" statements can follow from "false" premises. That was well known long before Godel.

7. Jun 9, 2009

### friend

Yes, a statement can be true irrespective of whatever anything else might say (true or false). Here we have a proposition (P) constructed with the symbols of a system (S). And P can be true regardless of what S says. This is the situation also if S implies P, right?

8. Jun 9, 2009

### sylas

That doesn't make any sense. If S is a "system", then you can't say S implies P. You can say S proves P, but that is a different statement entirely.

It looks to me that you don't actually know anything about how Godel's theorem is actually proved. If so, you'd do a lot better to learn a bit more about the theorem.

There's a really lovely little book by logician Raymond Smullyan, called "Forever Undecided", which gives a discussion of an equivalent of the Godel theorems for a simple propostional local of belief. Smullyan is great for learning about logic and having fun while doing it.

9. Jun 9, 2009

### friend

I thought the word "proves" was just another say of saying "implies". What is implied that isn't proved? And what is proved that is not implied?

10. Jun 9, 2009

### sylas

Informally, yes, they can be used in similar ways.

But you are speaking here specifically of formal logic, where there are very precise meanings for things.

In formal logic, "implies" is propositional connective, just like "and" and "or". If P and Q are propositions, then "P implies Q" is also a proposition. Here are truth tables for and, or and implies:
$$\begin{array}{*{3}{c|ccc}} \wedge & T & F & \hspace{2em} & \vee & T & F & \hspace{2em} & \Rightarrow & T & F & \hspace{2em} \\ \cline{1-3} \cline{5-7} \cline{9-11} T & T & F && T & T & T && T & T & F \\ F & F & F && F & T & F && F & T & T \end{array}$$​

Proves, however, is a "meta-logical" relation, between a proof system, or a set of axioms, and a proposition. It is often written as follows:
$$S \vdash P$$​
This is not a "proposition" with a truth value, but a definite statement, that P can be proved from the system S.

11. Jun 9, 2009

### Phrak

It took me a while to understand what you meant. The addendum helped.
The game of symbols is the manner in which Godel constructed his proofs.

I have one slime text on Godel's Theorem, "Godel's Proof", Nagel and Newman, that states:

"Godel showed that Principia, or any other system within which arithmetic can be developed , is essentially incomplete. In other words, given any consistent set of arithmetical axioms, there are true arithmetical statements that cannot be derived from the set."

But this is wildly dependent on what one thinks and believes is true. It's an informal statement by the authors about adherence to absolute truths (with the T capitalized) that have not been revieled by the original set of axioms.

If 'true' is taken to mean logical truth value then something taken as an absolute truth effectively imposes additional constraints to the original set of axioms. That is, it adds axioms. One could have chosen to add an alternative axiom, consistent with the rest, yet incompatible with the first choice. I believe that 'truth' in a non absolute sense is what Godel intended, that the authors seem to have missed.

Last edited: Jun 9, 2009
12. Jun 9, 2009

### sylas

A more precise sense of what Godel proved is that for any logical system sufficiently expressive to represent arithmetic, one of the following two cases must hold:
• Every sentence is provable in the system. That is, the system is inconsistent.
• There are some sentences, where the system cannot prove the sentence OR prove its negation. That is, the system is incomplete.

The authors don't need to be assuming any absolute truth. In logic, "truth" is defined with respect to a model. You don't need an absolute model or an absolute truth. The incompleteness of a formal system means that for ANY model, there are statements true in that model which the system cannot prove.

Cheers -- sylas

13. Jun 9, 2009

### Phrak

As I have been saying, you first presume your model to hold absolutes for which your system must comform or it is incomplete.

14. Jun 9, 2009

### sylas

And I responded to what you are saying because it is incorrect. I think you have misunderstood how this works.

A system is incomplete not with respect to a single model, but with respect to ALL models, because there are sentences for which it can neither prove the sentence nor its negation. It is wrong to think you first presume a particular model. You don't.

Incompleteness of a system is property of a system itself, independent of models.

Cheers -- sylas

15. Jun 10, 2009

### Preno

How is it wildly dependent on what one thinks and believes to be true? The only thing it depends on is the consistency of PA. Assuming that, the statement is true because of how it is constructed: G is true iff it is unprovable in PA, and if PA is consistent, then it indeed is unprovable and therefore true.

eta: if you want to know how I know that PA is consistent - since I know the axioms are all true, formal inferences preserve truth and a statement and its negation cannot simultaneously be true, it follows that we cannot prove both a statement and its negation from PA. (Also, if you wish to identify truth with provability in PA or some stronger system, you don't really have the option of doubting the consistency of PA, anyway.)

And of course a lot of other things that don't follow from PA are true. For example, the fact that the integers are well-ordered.
Yes, since Godel's sentence is undecidable, the theory remains consistent whether you add G or ~G as an axiom. That doesn't necessarily mean much, though. PA remains consistent even if you add a constant c and axioms c>1,c>2,c>3,... Consistency isn't everything, if you want the axioms to be true, you also need to demand omega-consistency. PA+~Con(PA) is consistent, but not omega-consistent, and therefore not true (where Con(PA) is the undecidable arithmetic statement coding the fact that PA is consistent).
Godel was a well-known Platonist, he certainly did believe in absolute mathematical truth.

Last edited: Jun 10, 2009
16. Jun 10, 2009

### friend

It would seem the inherent truth of arithmaical statements is imposed from outside that system. We recognize the truth of those statements intuitively. But those statements in and of themselves do not equate to True or False. I mean whoever seen something like (2+3=5)=True, or (2+3=6)=False? We simply intuit the truth of 2+3=5. So when the logical truth values of True and False are not even part of the formal language of the system in question, like arithmatic, then it is trivially obvious that the system can't prove any statement True or False. What?

17. Jun 10, 2009

### sylas

No, we don't. These are theorems of the formal system, which must be provable. The actual way of writing meta statements in logic is more like this. To say 2+3=5 is "true" in a model M, we write:
$${\cal M} \models 2+3=5$$​
If the model is implicit in context, then you can just write:
$$\models 2+3=5$$​
This is pretty much what you think of as "(2+3=5)=TRUE", except that that there is a clear difference between the formal language of arithmetic, and the formal meta-language for expressing whether sentences of arithmetic are true, or provable.

To say 2+3=5 is "provable" in a system S, we write:
$${\cal S} \vdash 2+3=5$$​

Note that there's a clear difference between "true" and "provable", which is important when you are looking at properties of formal system, like completeness or consistency.

You seem to be making assertions about logic without actually knowing any formal logic. You are mixing up models and axioms and sentences of the logic and statements about the logic, and don't even seem to be aware that you are doing it.

There's no point worry about Godel until you do a bit more basic introductory formal logic.

Formal systems do prove things just fine.

Cheers -- sylas

18. Jun 10, 2009

### friend

Yes, at some point I might have to educate myself in these intricacies. At present I know propositional calculus and predicate logic. I wish I had more time to continue my studies. Where do you suggest I learn the rest?

I understand Russel and Whitehead tried to prove that math was complete by showing how it could be derived from propositional logic. Is that right? I understand their derivation went through the first order predicate logic of set theory. But to even recognize the truth that an element belongs to a set is imposed and not derived, right?

I think in order to prove that arithmetic statements are true, one would have to derive math statements from propositional logic, since propositional logic is the algebra of true and false. Otherwise, the truth of the math statements are merely recognized intuitively by the reader and not derived by algebraic manipulation of the system's axioms. And if the truth of statements is not derived from the axioms of the formal system, then of course the truth of statements is not connected to its provability.

Did Russel and Whitehead actually succeed in deriving math from propositional logic?

Last edited: Jun 10, 2009
19. Jun 10, 2009

### CRGreathouse

You need a lot more than propositional logic: you need predicate calculus and set theory. Have a look at Metamath for some examples of derivation of mathematics from the axioms themselves.

20. Jun 10, 2009

### sylas

As has been pointed out, they needed a lot more than propositional logic. They did show that formal logic could be used effectively as a basis for mathematics.

Godel showed that the formal systems of Principia could not be complete and consistent.

Last edited by a moderator: Jun 11, 2009