The Rebbe taught this discourse on Shabbat Parshat Shelach in 1974.  It concerns the story of the Spies entering the Land of Canaan.  The discourse expolains that they have a mission to elevate the Land, to reveal a special positive dimension. But the spies were reluctant to get involved in the lowly aspects of the Land. They preferred the ethereal life in the Desert.  In the life of the individual, the Land represents the practical Mitzvot. We 'descend' to carry out the Mitzvot and thereby elevate the world.