Beyond the senses is the mind, beyond the mind is the highest (created) Being, higher than that Being is the Great Self, higher than the Great, the highest Undeveloped.

In plain words: Death, the teacher, lists layers from outer to inner. The mind is deeper than the senses. Deeper than the mind is the first created Being, then the Great Self, and deeper still is the Undeveloped, the hidden root of the world.