The Talmud tells us that when Alexander the Great reached the gate of Gan Eden, he called out, "Open the gate for me!" To his chagrin, he was turned away with the response, "This is the gate to Hashem, only the righteous may enter."
Alexander retorted, "I am a king of great renown. Since you refuse to open the gate, at least give me something from Gan Eden."
They gave him an eyeball. When he weighed it, stacking all of his gold and silver on one side of the scale and the eyeball on the other, he was shocked to discover that the eyeball outweighed all of his wealth.
He asked the wise men for an explanation and they said, "This is the eyeball of a human being. The human eye is never satisfied with what it sees; it always wants more. Nothing in the world is enough, for a person will crave whatever he sees. In fact, his eyes are not satisfied until the day he dies and his eyes are covered with earth."
"Can this be true?" Alexander asked. The wise men countered, "Cover the eye with a little earth. The true weight of the gold will reverse the scale immediately." (Tamid 32b) Hashem denied Alexander access to Gan Eden because of his greed and obsession with power. Bent on conquering the entire world, his desires were inflamed beyond all reason. When he demanded the reason for his rejection, he was given an eyeball, because the eye is the source of desire. Just as his desire was out of control, so too, the weight of the eyeball was completely out of proportion.
Heaven was hinting to him about the futility of chasing after earthly desires which can never be satisfied. But Alexander lacked the Torah wisdom to perceive this truth and could not see anything beyond his own ego and self-gratification. This mighty warrior conquered most of the world, yet he could not overpower his own eyes.