Faith vs Reason: Mutazilites, Ash'arites, et multae Caetera

I have just finished reading a book on Islamic law, that being the subject of one of the chapters of a book I am currently writing. One of the parts that caught my interest was the description of the disagreement between two philosophical schools. The Mutazilites, or rationalists, held that human reason was capable of recognizing good and evil; they concluded that although religious obligations, such as prayers, had to be based entirely on revelation, other legal and moral rules did not. The Ash'arites, on the other hand, held that human reason was unable to make such judgements, hence all rules had to be entirely based on revelation, with reason limited, if I correctly understand their position, to interpreting the meaning and application of what had been revealed.

It occurred to me that there was a small problem with the Ashirite position. If humans are entirely unable to distinguish good from evil, how can they distinguish God from the Devil? How, in other words, when a powerful supernatural being tells them to do something, can they tell if he is good or evil? The same problem must exist for other religions in which some theologians hold a position analogous to the Ash'arite. I am curious as to whether any of my readers can tell me how it is dealt with.

One of the other interesting details in the book was the attitude of Islamic legal scholars to probability. In their view, again as I understand it, as probability becomes stronger, it becomes certainty.

An example is the status of a hadith, an oral tradition about something Mohammed did or said. A hadith known through only one chain of transmitters is at best probable. A hadith known through N independent chains—legal scholars disagreed about the size of N—is certain.

That view of probability theory reminded me of something I had seen before. As anyone who has spent much time arguing with Objectivists, followers of Ayn Rand's philosophy, is likely to have discovered, they too believe in certainty where others would see, at most, a high probability.

I succeeded, with a great effort, in resisting the temptation to title this post "Was Ayn Rand a closet Muslim?"