Sefer HaBahir 15

1[Rabbi Bun continued,] "And to what is [the letter] Bet similar? To a person that it is formed with wisdom: closed on all sides but open in its front. [But] the [letter] Aleph is open from its back. This is to say that the tail of the Bet is open from behind, and if it were not thus, humans would not be sustained. So too, if it were not for the Bet in the tail of the Aleph, the world would not be sustained."