The nine discourses in the Hexæmeron all shew signs of having been delivered extempore, and the sequence of argument and illustration is not such as to lead to the conclusion that they were ever redacted by the author into exact literary form.  We probably owe their preservation to the skilled shorthand writers of the day. cf. Letterccxxiii. § 5, p. 264.  It is believed that tachygraphy was known from very early times, and Xenophon is said to have “reported” Socrates by its aid.  The first plain mention of a tachygraphist is in a letter of Flavius Philostratus (A.D. 195).  It has been thought that the systems in use in the earlier centuries of our era were modifications of a cryptographic method employed by the Christians to circulate documents in the Church.  No examples are extant of an earlier date than the tenth century, and of these an interesting specimen is the Paris MS. of Hermogenes described by Montfaucon, Pal. Gr. p. 351.  The exact minutes of some of the Councils—e.g. Chalcedon—seem to be due to very successful tachygraphy.