This discourse, taught by the Rebbe in 1965 for Sedra Yitro, discusses Moses' climbing Mount Sinai after the revelation of the Ten Commandments.  He was going to be forty days and nights on the mountain.  At the beginning of his ascent the Torah describes him entering 'the thickness of the cloud'.  What does this represent?  The hiddenness of G-d in this world? The exalted essence of G-d which is described as 'dwelling in darkness'? Or both..?  The discourse also presents the way Moses draws close to G-d in order to draw with him every other Jew.