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

“In this degree it is that our attention is called to the fact that the Mason who has attained proficiency in this grade has been enabled to discover a sacred symbol, placed in the centre of the building, and alluding to the G.G.O.T.U. Doubtless we have often asked ourselves what that phrase and what that symbol imply. Need I repeat that the building alluded to is not the edifice we meet in, but is our own selves, and that the sacred symbol at the centre of the roof and of the floor of this outward temple is but symbolic of that which exists at the centre of ourselves, and which was spoken of by the Christian Master when He proclaimed that ‘the kingdom of heaven is within you’; that at the depths of our own being, concealed beneath the heavy veils of the sensual, lower nature, there resides that vital and immortal principle, which is said to ‘allude to’ the G.G. because it is nothing other than a spark of God Himself immanent within us.” [via]