That is the task of Arthur Melzer’s new book Philosophy Between the Lines: The Lost History of Esoteric Writing. According to Melzer, a professor of political science at Michigan State University, until about 1800 it was the worst-kept secret among philosophers that their colleagues throughout the ages had made a regular practice of concealing their true, “esoteric” beliefs underneath a surface of orthodox, “exoteric” pieties. This “double doctrine” would simultaneously appease the masses, to whom the writer would appear to uphold contemporary conventions, while remaining accessible to a few careful, worthy readers. As the Enlightenment pressed on, however, esoteric writing ceased, and with it esoteric reading—so much so that, by the mid-twentieth century, the whole idea was pronounced a myth.

Ian Tuttle in The University Bookman, Winter 2015

Never heard of this, but it sounds fascinating.