The relation of h to f. Head to body? Mind to flesh? Quite possibly.
He is head and flesh. She is society, head, and flesh.
Thus, I am going to try substituting h for f and r for l in words that come up.
One, immediately. Fellow becomes hero. For becomes hole, or haul. Part of the mystery of consonants is that they do have relations to one another. And the vowels are a universe.