The Deeper Symbolism of Freemasonry from The Meaning of Masonry by Walter Leslie Wilmshurst.

“It is after investment with the apron that the initiate is placed in the N.E. corner. Thereby he is intended to learn that at his birth into this world the foundation-stone of his spiritual life was duly and truly laid and implanted within himself; and he is charged to develop it; to create a superstructure upon it.” [via]