late 14c., "front;" variant of further (adj.). From 1510s as "additional;" 1560s as "more remote."
15c. alteration of Middle English ferther (c. 1300), a variant of further (adv.). There is no historical basis for the notion that farther is of physical distance and further of degree or quality.
the application of the law was extended farther
he is going no farther in his studies
moved farther away
the practice may go back still farther to the Druids
farther down the corridor
they live in the farther house
farther from our expectations
farther from the truth