Note 44 — Chapter XI · THE FOREIGNER
Reading edition
Feist, on the other hand (PBB 36. 329), makes the Kelts responsible for the shift from p to f, because initial p disappears in Keltic: but disappearance is not the same thing as being changed into a spirant, and there is no necessity for assuming that the sound before disappearing had been changed into f. Besides, it is characteristic of the Gothonic shift that it affects all stops equally, without regard to the place of articulation, while the Keltic change affects only the one sound p.