is Ibn Tufayl's Hayy Ibn Yaqzan. Ibn Tufayl was a 12th century Muslim philosopher, mystic, physician and court official. (In many respects, Maimonides is his Jewish counterpart, except that Maimonides was no mystic.)
It's the story of a man who is either spontaneously generated on a deserted island, or who washes up there in a sealed cask -- of course, no one knows. Raised by a gazelle, he progresses, by rational inquiry and deductive reasoning, to understanding that there is a Creator. Hayy sees his task in life as serving and getting as close to that creator as possible.
Ibn Tufayl's text made it to England in the 1600s, where it was translated and where it received favorable reception. It's very likely that Daniel Defoe was influenced by it in his writing of Robinson Crusoe.
There are many beautiful things about the story, and the story behind the story. The most intriguing aspect is that, when Hayy finally comes into contact with civilization and its organized religions, they are a huge disappointment to him. In the end, he returns -- with one disciple -- to his island, to live out his days in the unmediated closeness to God that he had come to realize through a lifetime of study, thought, meditation and experiment.
Best read of the 12th century, for sure.