Odysseus returns to Ithaca unrecognizable: Athena pours mist over him and then disguises him as an old beggar, so that neither his wife, nor the citizens, nor his friends recognize him before the suitors pay for what they did. The poem then turns recognition into a series, and each person recognizes him in a different way.
In the hut of Eumaeus, Telemachus is the first to know. When the beggar says "I am your father," the son refuses to believe and thinks a god is deceiving him so that he will weep more; he is convinced only when the man affirms that "no other Odysseus will come here." Later, in the palace, the dog Argos recognizes him. Lying on the dung at the door, he sees his master after twenty years, wags his tail and lowers his ears, but can no longer rise. Odysseus wipes away a tear, and the dog dies.
In Book XIX, the nurse Eurycleia recognizes him by the scar that a boar left on his leg, when she washes his feet. Odysseus had sat near the hearth, turned toward the shadow, precisely to avoid this; she drops his foot into the basin, the bronze rings out, and he forces her to silence. Later he shows the same scar to the swineherd Eumaeus and to the cowherd Philoetius, so that they will believe. Penelope, in turn, recognizes him by the olive-wood bed (Book XXIII), and Laertes, in the last book, by the scar and by the trees of the orchard.
In each case the sign is of a different kind: the instinct of the dog, the marked body, the secret of the room, the orchard of childhood. The name alone is not enough, because any liar could say it, and Odysseus himself often lies: to Eumaeus and to Penelope he tells Cretan tales. The lesson the poem repeats is that recognizing someone demands more than appearance: it demands the memory of things that only the two of them know.