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:

Wrong. The Islamic viewpoint is that there is truth and falsehood in the Bible. One of the purposes of the Qur'an is to indicate what is the truth and what is the falsehood. As such one can base arguments on the Bible.

If one is out to prove that something is wrong, then of course one uses the material available to do so. If one is to prove that something is correct, then you would use the other material available. Since the Bible contains both truth and falsehood, one can do both.

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:

Wrong again. The Qur'an makes clear that the peoples who had received revelation before have distorted those books, that is one of the reasons for the sending of the Qur'an, to guard what was sent before..ie. to be able to tell what is right and wrong in what is claimed to be the previous revelation. If the claimed previous revelation is not in accord with the Qur'an, then that part which is not in accord is wrong. The part which is in accord with the Qur'an is ok.

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