"Anyone who denies the law of non-contradiction should be beaten and burned until he admits that to be beaten is not the same as not to be beaten, and to be burned is not the same as not to be burned."--Avicenna
>If someone's believing a false premise is going to poison his work
>as irrational, then, of course, no one is rational. That's an awkward
>place to lead yourself.
Ah no, I never said that: Euclid thought that his axioms were immutably true, which doesn't count as a premise as we understand it.
"What is logical consistency, and why is it a necessary condition for doing mathematical work?"
Following the laws of logic: Non-contradiction and Excluded Middle (all else follows from there). It is necessary because without these, you either can do no work whatsoever, or can prove anything, depending on which way you take it. Also, there is the other fact that they are true.
There are two types of axiom, as I say again: the logical and the non-logical. In mathematics as in life you cannot break away from the logical axioms, but you can create different axiomatic systems using the non-logical axioms.
>There are, of course, further constraints on abstract speculation. One can explore an >axiomatic system for its own sake, but usually the question arises "What has this to do >with reality?" That is, we want to find an isomorphism between the results that arise in >our system (by means of applying rules of inference to our axioms in order to generate >theorems) and things that occur in the world. Sometimes the world intrudes even when >it was supposed to have been banished; recall what Gödel did to the Principia >Mathematica.
Firstly, when you ask that question, it ceases to be pure mathematics, and becomes science. Knowledge is a different concept in science, asking not "Is P true?" but "how likely is it that P is true?" You are trying to shift between two very different things all the time: between the logical and non-logical axioms and between the mathematical and scientific concepts of knowledge or truth.
What Gödel did is so often misunderstood I could tear out my own hair. He proved that: 1. Consistent sets of axioms in *arithmetic* cannot make any proposition provable. That means that you can have a proposition that is not contradictory to accept or deny. The absence of completeness is not a logical difficulty for mathematics, it doesn't make it irrational.
2. You cannot prove the consistency of a set of axioms, only its inconsistency. This means we have to be agnostic about this matter in arithmetic (note not geometry). Again, it doesn't cause any logical failure.
Number theory is a matter of logical consequences from axioms- we can do this in spite of Gödel.
>Now I'll turn to what I take to be the point of science (and mathematics is a type of >science): knowledge. Knowledge is justified true belief. Faith is belief without proof. To >make faith become knowledge, we have to add justification and truth to belief. Now, I >think you'll admit that there is at least one true thing. So if I believed that true thing, I >would have a true belief. Now, is the difference between a rational belief and faith that >the latter lacks justification? I don't think this can be definitively stated. Simply put, it is >possible for a belief to be justified without proof. If proof is not the sole means to >justification, then faith is legitimate.
Firstly, Mathematics is not science in the modern understanding. You are factually wrong there.
>How can a belief be justified without proof? Well, take Gödel's first incompleteness >theorem. This proves that there is some arithmetic statement the truth or falsity of >which cannot be proved by the axioms of arithmetic. In other words, there is NO WAY >to prove whether it is true one way or another. There are two alternatives. We could >simply suspend belief, like the ancient skeptics. We could, however, accept the truth of >the Gödel sentence without proof of it. Indeed, if we are to "take a side" regarding that >particular statement, we have to accept it without proof, because Gödel proved that >there never will be a proof available.
If something is unprovable, it means that its negation is contradictory, which means that it is consistent to accept it as true or false, and must be considered an axiom. We can prove that something is like this (take whether or not there exists and infinity between the number of reals and the number of integers as an example of this). If, however, we take a side based on faith, we will deny a mathematics that is consistent, valuable, beautiful, aesthetic, wonderful and magnificent. What's more, when you deny it you are actually wrong to do so. If you say "P is true, not P is false" rather than "P can be an axiom or not P can be an axiom", you are wrong.
However, that is anyway irrelevant, because the world is not a defined axiomatic system in the same way that mathematics is. The question of whether a religious belief is true or not is not mathematical, it is scientific: We couldn't define as axiomatic that God exists, that would be absurd, it would be an attempt of man's to definine reality. This of course renders your last paragraph obsolete.