Joseph brought down to Egypt is the higher self descending into the body, the bright inner faculty carried down out of its own country into the dense land of matter. Egypt is the flesh, the realm of the senses and of bondage where the spiritual nature is sold into service to the appetites that govern there.
Being bought by the captain of the guard is the taking-over of that descended self by the ruling powers of the body, put to work under the master of the physical house. The going-down is the necessary immersion of spirit in the earthly condition, where the finer awareness must live captive for a while inside the very flesh it came to redeem.
Joseph: the higher self, the bright inner faculty carried down into the body
Egypt: the flesh and the world of the senses, the land of matter and bondage
Potiphar, captain of the guard: the ruling power of the body that takes the descended self into its service