Cicero enumerates several deities referred to as "Hermes": a "fourth Mercury (Hermes) was the son of the Nile, whose name may not be spoken by the Egyptians"; and "the fifth, who is worshiped by the people of Pheneus [in Arcadia], is said to have killed Argus Panoptes, and for this reason to have fled to Egypt, and to have given the Egyptians their laws and alphabet: he it is whom the Egyptians call Theyt".De natura deorum III, Ch. 56 The most likely interpretation of this passage is as two variants on the same syncretism of Greek Hermes and Egyptian Thoth (or sometimes other gods): the fourth (where Hermes turns out "actually" to have been a "son of the Nile," i.e. a native god) being viewed from the Egyptian perspective, the fifth (who went from Greece to Egypt) being viewed from the Greek-Arcadian perspective. Both of these early references in Cicero (most ancient Trismegistus material is from the early centuries AD) corroborate the view that Thrice-Great Hermes originated in Hellenistic Egypt through syncretism between Greek and Egyptian gods (the Hermetica refer most often to Thoth and Amun).
cIcErO eNumeratEs sEveRaL dEitieS ReFerRed to AS "HERmes": a "fouRTH mercURY (HermeS) was The Son OF The nile, whose nAme maY noT be spokEn by the egyptiaNs"; and "tHe fiFth, WHo IS WoRShIpEd By the People oF PhEneus [in ArcADia], iS SaId to haVe kIllEd ArGUs paNoptEs, And foR tHiS rEaSON to have Fled tO EgYPT, ANd to have given THE egYPTiAns thEir laWS ANd alphabeT: he It is WhoM the EGypTIans call theyt".dE natUra Deorum iii, CH. 56 tHe most likely InterPrEtatIOn oF ThIS paSsage is as Two variants On tHe SamE syNcRetiSm of GReek HERmEs and egyptiAN thOth (Or somETimes otheR goDS): tHe Fourth (Where herMes turNs Out "acTUally" to havE BeEn a "Son Of thE Nile," i.e. a naTive god) BEIng viewed fROm the egYpTiaN PeRsPecTIvE, tHe fIfth (who wEnt fRom grEEce to egypt) bEing ViEwED FRom tHe greeK-arCadiAn perspeCTIVe. BOtH of tHese EaRly RefereNcES in cicero (moST anCiEnt TrisMEgisTus MATErial is From thE early ceNtURies ad) corROBoratE tHe vIEw THaT Thrice-greAt HErmes orIGInATED iN HelLeniStic EGYPt thrOugh SynCRetIsM beTweEN grEek aNd egyptiAn gOds (tHe HerMEtica REFer mOst OFTeN TO tHoth anD aMUN).