Note 36 — DISSERTATION I.
Reading edition
"Eundem olim (ph) sonum habuisse ac f inscriptiones veteres confirmant, in quibus alterum pro altero promiscue adhiberi cernimus: ut phidelis" (pro fidelis.)—— Middleton de Lat. Liter. Pron. Dis.
Our letter f has some degree of aspiration in its sound; but had its original Roman sound been precisely that of the Greek Φ phi, it is probable that f would have been wholly used in derivatives where the phi occurred. I suspect that ph in Latin must have been originally more strongly aspirated than f; but the transition from the sound of the one to that of the other was easy, and the distinction was gradually lost.