In order to understand the structure of the _Iliad_, we must keep fast hold of the guiding clue which is supplied by the author in the first line of his poem. The subject, he tells us, is the _Wrath of Achilles_. The motive of the greatest of epics is _wrath_--blind, unreasoning fury, which knows no law, and acknowledges no right. Keeping this in view, we are able to explain what seems at first sight to be a strange anomaly in the conduct of the story--the absence of the hero from the scene of action during three-fourths of the narrative. For Achilles is not less the hero of the _Iliad_ than Odysseus is the hero of the _Odyssey_, and in both cases the character of the man determines the structure of the poem. Odysseus is a man of middle age, in the maturity of his splendid powers, with his judgment refined by experience, and his passions cooled by time. From the moment when he sets sail from Troy he remains faithful to the fixed desire of his heart. All the malice of Poseidon, all the spells of Circe, all the loveliness of Calypso, cannot shake him from his resolve to return to his home in Ithaca, and live out his life in calm domestic happiness and peace. Yet he is entirely free from the narrowness which commonly belongs to a fixed idea. He knows the uncertainty which attaches to all human hopes, and is as ready to enjoy the passing hour as the youngest sailor of his crew. He has the hungry intellect, which would fain take all knowledge into its compass, and the spirit of soaring enterprise, which delights in discovery and daring adventure. But above all he has the patient, constant human heart, faithful through all turns of fortune to one sober ideal. It is this steadfastness of purpose and sweet reasonableness in the hero which gives to the narrative of the _Odyssey_ its smooth and pellucid flow, and makes it the most delightful of all story-books. Achilles, on the other hand, is the incarnation of the spirit of youth, with its passionate pride, its acute sensibility, and its absorption in self. He is like one of the great forces of nature--unreasoning, elemental, mighty to create or destroy. His inaction is as tremendous as his action. He is offended, and the Greeks, deprived of his aid, are brought to the brink of ruin--his friend is slain by Hector, and the current of his fury, thus directed into a new channel, sweeps the whole Trojan army before it in havoc and rout.