 It is likely that in some cases letters were forged in order to authenticate these forged treatises. Thus a correspondence between Plato and Archytas dealing with the acquisition of the writings of Ocellus (Diogenes Laertius VIII 80–81) may be intended to validate the forgery in Ocellus’ name (Harder 1966, 39ff). A letter from Lysis to Hipparchus (Thesleff 1965, 111–114), which enjoyed considerable fame in the later tradition and is quoted by Copernicus, urges that the master’s doctrines not be presented in public to the uninitiated and recounts Pythagoras’ daughter’s preservation of his “notebooks” (hypomnêmata) in secrecy, although she could have sold them for much money (see Riedweg 2005, 120–121). Burkert (1961, 17–28) has argued that this letter was forged to authenticate the “Pythagorean Notes” from which Alexander Polyhistor (1st century BCE) derived his influential account of Pythagoreanism (Diogenes Laertius VIII 24–36 — see the end of this section and for Alexander see section 4.5 below). While some of Pythagoras’ teachings were undoubtedly secret, many were not, and the claim of secrecy in the letter of Lysis is used to explain both the previous lack of early Pythagorean documents and the recent “discovery” of what are in reality forged documents, such as the notebooks.