Campbell on Muhammed:
There is indeed one miracle, and but one, which he often urges against the infidels, as the main support of his cause; a miracle for which even we, in this distant region and period, have not only the evidence of testimony, but, if we please to use it, all the evidence which the contemporaries and countrymen of this military apostle ever enjoyed. The miracle I mean is, the manifest divinity, or supernatural excellence, of the scriptures which he gave them; a miracle, concerning which I shall only say, that as it falls not under the cognizance of the senses, but of a much more fallible tribunal, taste in composition, and critical discernment, soa principl eof less efficacy than enthusiasm, even the slightest partiality, may make a man, in this particular, imagine he perceives what has no reality. Certain it is, that notwithstanding the many defiances which the prophet gave his enemies, sometimes to produce ten chapters, sometimes one, that coudl bear to be compared with an equal portion of the perspicuous book, they seem not in the least ot have been convinced that there was any thing miraculous in the matter. (A Dissertation on Miracles, p. 67)
What Campbell is talking about here is what is often called in Muslim thought simply "Tahaddi" (Challenge). In several passages of the Quran, Muhammed challenges his pagan opponents to imitate the Koran (e.g., Sura 2:23-24; Sura 10:38; Sura 11:13; Sura 52:33-34; cf. also Sura 17:88). This was closely associated with what was called ijaz (incapacitation), the chief miracle of Muhammed's prophethood. In the Mutazzilite tradition, this was understood as a miraculous incapacitation of Muhammed's contemporaries to produce a Sura in response to the Challenge that was similar in content and style to those of the Quran; however, the notion of ijaz began to be applied more generally to the inimitability of the Quran; as Sura 17:88 says, not even the cooperation of djinn and humans could produce a similar Quran. (On one reading of Sura 28:48-49, and in what seems to be the majority of Muslim commentators, ijaz is extended to at least some of the chapters of the Torah.)
I'm inclined to think that this argument is worth taking a little more seriously than is sometimes done. The most common complaint is that it is purely subjective. I'm not inclined to regard this as a serious complaint, in part because I think a theory of taste can be developed that allows for at least stable inter-subjective agreement. It's also commonly objected that beauty is not necessarily an indication of truth; but sometimes, of course, it seems to be, or at least one form of beauty seems to be. Such objections need to be more detailed if they are to work. I've always actually been inclined to believe that perhaps the strongest Muslim argument for the inspiredness of the Quran was that an illiterate merchant in a backwater tribe could dictate a book of profound poetic beauty capable of changing lives and charging a civilization; and this sort of argument would clearly be related to the issues of Tahaddi and ijaz. The chief difficulty of the whole matter is pinning down what exactly the challenge is and what criteria would allow one to decide the matter. For further information, see this page, which contains several attempts to formulate the Challenge, along with Christian responses.