Gift of Men
In J. R. R. Tolkien's stories set in Middle-earth, the Gift of Men is a gift of the deity Ilúvatar to his Younger Children, which remains a source of some confusion for Tolkien enthusiasts. The concept includes both mortality and free will. Below are two interpretations of the nature and extent of the Gift of Men as articulated by Tolkien.
A spiritual/theological view of the Gift of Men
The Gift of Men was an act of the supreme being Ilúvatar that set the race of Men apart from the Elves. While the race of Elves would know the most bliss and contentment and would conceive more beauty than any others of the Children of Ilúvatar, it was decreed by this gift that Men would be the prime instruments of Ilúvatar within Arda.
Ilúvatar willed that the spirits/hearts of Men are not content within Arda, and find no rest therein, and therefore seek beyond the world and its confines. They are not bound to the Circles of the World, as the Elves and all other creatures of Arda are bound to the Earth. The spirits of Men truly leave the physical world, and do not return. Thus their fates are completely sundered from that of the Elves, who do not die until the world dies; if slain by violence or ill chance, or by wearying at the last of the passage of centuries, they are gathered in the Halls of Mandos. But as the years grow long and Time wears, even the Valar will come to envy the gift of Ilúvatar to the race of Men, that of liberation from the physical world, and the inevitability of loss and sorrows that must come with this existence within Arda.