At the moment that Philinte is making advances to Eliante, Alceste enters ranting that Célimène has been unfaithful to him. He claims to have in his possession a love letter which Célimène has written to Oronte. Philinte tries to calm him, saying that the letter simply might have given the wrong impression, but Alceste refuses to listen. Then Alceste offers his devotion to Eliante in order to get vengeance on Célimène. Eliante stays him with the words: "guilt in loved ones soon turns to innocence again, resentment quickly vanishes; we all know what lovers' quarrels are." Alceste claims undying antagonism toward Célimène.
Alceste's entrance on the scene emphasizes again that he is the "spleen" character, a man given to obsessions and intemperate behavior. He enters in a rage and has to be calmed by Philinte. His language is comically hyperbolic, and his actions are similarly exaggerated. Feeling betrayed by Célimène, he immediately proposes to Eliante, saying: "Accept my heart, Accept it, in that faithless woman's place; Only in that way will I be avenged. I'll punish her by the sincere attachment, profound affection, worshipful attentions, eager devotion and assiduous service my heart will henceforth offer at your shrine." This honest man is driven to extremes in promising to idolize one woman simply to make another jealous. This is incongruous with the rational behavior Alceste has been advocating throughout the earlier parts of the play. The rational misanthrope is now seen as the not so admirable man of confused ideas.