We use “farther” and “further” in slightly different situations.
Farther: physical distances
Hartford is close; New York is farther.
Further: figurative or metaphorical distances
I want to think about this question further.
Occasionally, though, it gets a little confusing. Check out this scene from the movie Labyrinth:
Here's the actual text:
Jareth: Do you still want to look for him?
Sarah: Is that... the castle beyond the Goblin City?
Jareth: Turn back, Sarah. Turn back before it's too late.
Sarah: I can't. Don't you understand that I can't?
Jareth: What a pity.
Sarah: It doesn't look that far.
Jareth: It's further than you think... and time is short. You have thirteen hours in which to solve the Labyrinth, before your baby brother becomes one of us...forever. Such a pity....
Now wait a second. Sarah says “[i]t doesn’t look that far” to the castle, (physical distance) but Jareth says, “It’s further than you think” (metaphorical distance). What’s going on here?
- The moviemakers messed up.
- Jareth is suggesting that the journey to the castle can’t be measured in miles alone. It’s also a metaphorical quest that will require Sarah to explore new places within herself.
So which is it, Jareth? And what should you do in your own writing?
In situations like this - in which the speaker's meaning is ambiguous - it doesn't matter. If you’re uncertain, use “further” – but don’t worry about it too much. People have been mixing these words up for hundreds of years, and these days, grammarians emphasize the distinction less than they used to. (Plus, the Brits use “farther” and “further” for physical distance. So maybe that’s what was going on in the movie.)