for the inference drawn from such a proof—one that is valid only when its premises are sound—collapses whenever its premise can be deemed sound only on condition that its conclusion is unsound, while the conclusion itself entails the unsoundness of the premise. To affirm both the soundness of the premise and the conclusion simultaneously is therefore to combine two contradictories.

Likewise, anyone who appeals to a passage of the Book to uphold something that contradicts what the Book itself states—such as the Christians who cite certain verses to vindicate their religion—has fallen into contradiction. If their argument were accepted, the Book would be praising their religion while also condemning it; that would make the Book self-contradictory, and a self-contradictory book cannot be the Book of Allah.

Should either side of the assertion prove false—whether the falsity lies in their religion or in the supposed praise—the Book would still contain falsehood, and a book that contains falsehood cannot be the Book of Allah. On both alternatives, then, it would follow that it is not the Book of Allah, and therefore it cannot be cited as evidence on the grounds that it is Allah’s own report. Nor can it yield certainty merely on account of the speaker’s being a wise and learned man, for no one is infallible except the prophets, peace be upon them.

The Christians, for their part, allow that someone other than the prophets might be infallible; even if we granted that premise, it would rebound against them. If they say, “He is a learned man but not a messenger from Allah,” we reply: his statement cannot be authoritative proof, for he may err—it can only serve as supporting testimony. But if he claims that Allah has sent him, while in fact Allah has not sent him with the entirety of this Book, then he is a liar, and nothing he says can be taken as proof; such a person cannot be considered upright.