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

“But in order to find the ‘perfect points of entrance’ to this secret (and we are told elsewhere that ‘straight is the way and narrow the gate, and few there be that find it’) emphasis again is laid in our teaching upon the necessity of complete moral rectitude, of utter exactness of thought, word and action, as exemplified by rigid observance of the symbolic principles of the square, level and plumb-rule.” [via]