I keep hearing creationists claim that 'logic' is cyclical because it relies on logic to demonstrate its validity.

How do we answer these guys?

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There's always this little tidbit:

Great visual Loren!

The problem is: the creationists (I'm thinking of Hovind jnr) seem to accept that their argumentation/hypotheses about god are cyclical. And they don't care (because they have faith).

They're saying that logic is as cyclical as faith-based hypotheses. They're trying to say that logic/reason and theism/faith are equally cyclical (because logic requires logic to prove its validity), therefore one should be able to choose between two equals...

Of course, I don't believe they're right by any stretch of the imagination. I just need to know how we demonstrate that logic doesn't require logic to demonstrate its own validity.

I assume by "logic" they mean reason and rationality, rather than just formal logic.  I think they may be taking Hume's observation that inductive reasoning is circular and running with it.  This is a genuine philosophical issue, although not really a practical one.  Applying it to reason as a whole is not valid because there is nothing outside reason to which you would want to appeal to break the circularity.  For example, to justify induction, you would appeal to deduction.  To justify the Bible or Koran, you would appeal to everything that isn't the Bible or Koran (which is quite a lot). But are your Creationists really asking for an illogical justification for logic?  An irrational justification for rationality?  An insane justification for sanity?  I mean that's easily done if that's what they want--"I have baloney in my shoes, therefore logic is valid".

I have noticed that I, and other sensible people, :-) , use circular logic of a kind, when we discuss biology. If a certain biological phenomenon is to be explained, we resort to evolution to explain it - or we might seek to explain it in terms of evolution. Is this assuming the point in question, ie. is evolution the truth about biological origins? If we can explain something in evolutionary terms, does that make evolution the truth?

I'd say the answer is NO. Yet I accept that evolution is true. How come? Well, it's because evolution has been demonstrated to be true. We have seen it in action:

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Test Tube Yeast Evolve Multicellularity

http://www.scientificamerican.com/article.cfm?id=test-tube-yeast-ev...

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Nylon-eating Bacteria

http://en.wikipedia.org/wiki/Nylon-eating_bacteria

Nylon-eating bacteria are a strain of Flavobacterium and the enzymes used to digest the man-made molecules became collectively known as nylonase.

{Note: nylon has only been in existence for a short time}

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Ring Species

http://en.wikipedia.org/wiki/Ring_species

a ring species is a connected series of neighboring populations, each of which can interbreed with closely sited related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene flow between each "linked" species

{Note: This demonstrates one mechanism for speciation}

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Evolution has been tested and re-tested, and never failed to be able to account for life as it is. Once that has been established, we surely don't need to re-invent the wheel? Once satisfied that evolution is demonstrably true, then surely we can look to it to help explain as yet unexplored biological phenomena, without having to resort to proving it all over again. In fact it makes no sense to have to continually demonstrate the truth of evolution, before making use of it.

Does that put the question of evolution 'to bed' - so to say? It does for me.

I would say that the same applies to logic. In its roots, logic is demonstrably true. If you smash a porcelain vase, it will not reform back into the vase it once was. It can be repaired, but there will be evidence of it. It can be ground down and used to make a new vase, but it will never be the vase it once was. This is demonstrably true.

So if you find a whole vase, it is logical to say that it, (THIS vase), has never been smashed. The logic here is based on what is demonstrably true. Isn't all logic built up from demonstrable truths at their foundation? And isn't it just plain wise, to use logic when it applies, rather than going back and justifying the whole box-and-dice, just to make a new point?

[Logic relies on]-[logic relies on]-[logic relies]-[on demonstrable truths].

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