Bacon sometimes, as in Valerius Terminus, wrote his doctrines in a purposely abrupt and obscure style, such as would “choose its reader.” He did not give his philosophy in a form which “whoso runs may read,” and was scornful of “the general reader.” But there is not the slightest ground in his works for believing there was a cipher in them. Nay, rather, he apologised for introducing ciphers as a part of learning at all. His connection with Essex, with his brother Anthony, with so many treasonable and state affairs, must have taught him the value of thoroughly understanding the powers of concealment in writing; and we are not surprised he considers ciphers in his general survey of learning. But he gives them no prominence.

In the 6th Book of De Augmentis, Chapter i., Bacon treats of ciphers and the method of deciphering. “Communications may either be written by the common alphabet (which is used by everybody), or by a secret or private one agreed upon by particular persons, called ciphers. There are many kinds, simple and mixed, those in two different letters; wheel-ciphers, key-ciphers, word-ciphers, and the like. There may be a double alphabet of significants and non-significants. The three merits of a cipher are: 1st, easy to write; 2nd, safe, or impossible to be deciphered without the key; 3rd, such as not to raise suspicion.” “Now for this elusion of inquiry there is a new and useful contrivance for it, which, as I have it by me, why should I set it down among the desiderata, instead of propounding the thing itself? It is this—let a man have two alphabets, one of true letters, the other of non-significants, and let him unfold in them two letters at once, the one carrying the secret, the other such a letter as the writer would have been likely to send. Then if any one be strictly examined as to the cipher, let him offer the alphabet of non-significants for the true letters, and the alphabet of true letters for the non-significants. Thus the examiner will fall upon the exterior letter, which, finding probable, he will not suspect anything of another letter written.”