Another book, the 13th-century Kabbalistic text Sefer HaTemunah, holds that a single letter of unknown pronunciation, held by some to be the four-pronged shin on one side of the teffilin box, is missing from the current alphabet. The world's flaws, the book teaches, are related to the absence of this letter, the eventual revelation of which will repair the universe. Another example of messianic significance attached to the letters is the teaching of Rabbi Eliezer that the five letters of the alphabet with final forms hold the "secret of redemption".