From his earliest works, Messiaen used non-retrogradable (palindromic) rhythms ("Example 2"). He sometimes combined rhythms with harmonic sequences in such a way that, if the process were repeated indefinitely, the music would eventually run through all possible permutations and return to its starting point. For Messiaen, this represented the "charm of impossibilities" of these processes. He only ever presented a portion of any such process, as if allowing the informed listener a glimpse of something eternal. In the first movement of "Quatuor pour la fin du temps" the piano and cello together provide an early example.