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

“These Mysteries were formerly taught, we are told, ‘on the highest hills and in the lowest valleys,’ which is merely a figure of speech for saying, first, that they have been taught in circumstances of the greatest seclusion and secrecy, and secondly, that they have been taught in both advanced and simple forms according to the understanding of their disciples.” [via]