Can we argue from a corrupted source?

I claim that it is a logical fallacy when Muslims base arguments for the truth of Islam on the Bible which in their mind is a corrupted source.

But to this objection one can often get this Muslim response:

But how do you decide which is the truth and the falsehood in the first place?

No, you have not understood how flawed this approach is. You are employing circular reasoning. You want to show the truth of Islam / the Qur'an. But since the truth of Islam is the goal of your reasoning you cannot use it in the process or for the foundation of your argument as well.

But in the above approach you do exactly that, because you use the Qur'an to decide which part of the Bible is right and which is wrong. That makes your argument circular and therefore wrong. If you have already declared that the Bible is corrupted and contains falsehood then EITHER [if not using the Qur'an at all] you base your arguments on something that might not be true, and your argument itself is therefore invalid, because it is reasoning from unverified and possibly false premises. OR you try to separate the true from the false [according to Islam] in order to get "pure" premises, but then you use the Qur'an to do so and make your argument circular which invalidates it too.

I agree that it is valid to use the Bible for the attempt to prove Christianity wrong. But if you want to postulate any corruption in the Bible [as most Muslims do], then the Bible cannot be used anymore for a validation of the Qur'an without committing one of the above logical fallacies.

I insist on it. You will have to come up with an independent validation of the Qur'an FIRST. THEN you can use the Qur'an to separate the true from the false in the Bible. BUT this independent validation of the Qur'an hasn't made its introduction to me yet. THAT is what you have to show me first.

But on the other hand, if you discard the Bible, then you are pulling the rug under the Qur'an since the harmony with the earlier revelations is one of the biggest reasons the Qur'an brings for its own validity. And you just discarded it!

This objection is met with:

This is valid ONLY if you have ALREADY shown the Qur'an to be right. WHERE have you or anybody done so?

BUT even your claim above is wrong. I have asked a many times: WHERE does the Qur'an speak of the textual corruption of the Christian scriptures at all? NOWHERE. This is a popular Muslim myth. I have posted the proof of it several times on this newsgroup. Please go and read the complete discussion of ALL Qur'an verses about the Jewish and Christian scriptures at this web page and then tell me whether Dr. Campbell forgot even one verse and if that one or any of these verses discussed speaks of corruption of the text of the Christian scriptures.

If not, then please stop making these unsubstantiated claims.

And I have to reiterate my argument from above: It is the as yet unvalidated Qur'an which claims to be in accord with the earlier scriptures. If this claim of harmony turns out to be false, then it is the problem of the Qur'an and not the problem of the Bible. It is the Qur'an which claims harmony and it isn't there, so the Qur'an claims something that is wrong. I cannot understand that it is so hard to comprehend that if the Qur'an makes false claims that this is a problem for the Qur'an and NOT for the Bible.

Basically Muslims say:

I hope expressed in this form you see how illogical this conclusion is. If a Bahai or any other newer religious descendent of Islam would reason that way in regard to contradictions between his book and the Qur'an, you would be the first to detect the error in this "logic".

Logical Fallacies of Muslim Apologetics
Authenticity of the Bible
Answering Islam Home Page