The paper knife represents the futility of any of the character's efforts to cut through what each is, or to terminate their existence in the room. After Garcin in his frustration shouts for the door to open, and it does, he then is paralyzed with the fear of the unknown and cannot leave. He stays because Inez knows what it is like to be a coward. Likewise, Estella and Inez do not act. Garcin tells Inez,
Yes. You, anyhow, know what it means to be a coward.
Shortly thereafter, Inez remarks, "We're--inseparable!" They understand each other; they see each other for what they truly are. The verity of Inez'a statement is emphasized by Estella's attempt to stab Inez and kill her with the paper knife, and then her desperate attempt to kill herself. Laughing, Inez sums up their situation in her realization of its finality:
Dead! Dead! Dead! Knives, poison, ropes--all are useless. It has happened already, do you understand? Once and for all. So here we are, forever.
Garcin stops his laughing, slumps onto his sofa: "Well, well, let's get on with it....." Certainly, the paper knife, symbolic of the futility of the three characters to change themselves and dissemble, underscores the theme of Jean-Paul Sartre: "Hell is---other people!"