Abu-Bakr, `Umar, `Uthman, and `Ali-the "orthodox caliphs" (A.D. 632-661). In the Arabic, the word orthodox in this phrase actually means mature, well-guided, correct; the usage in this book follows that of Western scholars, who have long written of the "orthodox caliphs." The reason these four caliphs are considered thus by Muslims is that, having known the Prophet personally and lived so closely according to his principles, they are looked to as great authorities and their decisions are considered precedents.