Legend has it that when the conqueror Amr ibn al-As entered Alexandria in 642, he ordered Caliph Umar I to set fire to the library's books. The story has been discredited, but Umar's apocryphal response deserves to be quoted because it echoes the curious logic of every book burner then and now. Umar acquiesced by saying, "If the content of these books agrees with the Holy Book, then they are redundant. If it disagrees, then they are undesirable. In either case, they should be consigned to the flames." Umar was addressing, somewhat stridently it is true, the essential fluidity of literature. Because of it, no library is what it is set up to be. Even within the strictest circumscriptions, any choice of books will be vaster than its label, and an inquiring reader will find danger (salutary or reprehensible) in the safest, most invigilated places.