Fascinated by the exploration of timbre through numeric tools, Jonathan Harvey made use of two concrete materials in Mortuos Plango, Vivos Voco: the sound of a bell and a child's voice. Composed for tape alone, this work has become a true classic of computer music. Relying on a rather complex musical thought, the composer nourishes his musical discourse with processes derived from the computer resources then available at the IRCAM. Far from any fruitless formalism, Mortuos Plango, Vivos Voco also reflects spiritual concerns of the English composer. The combination of spectral and post-serial influences leads to a true aesthetic of ambiguity. Following a study commissioned by the IRCAM, during which a fruitful relation with the composer took place, this paper proposes an in-depth analysis of Mortuos Plango, Vivos Voco and aims at showing its esthetical promises.