If a guy is feeling up a girl by running his hands up her legs, do you use the word "farther" or the word "further" to indicate his progress. I know the general distinction, but it seems to me that in this particular case, both are correct, since one is about distance and the other amount or degree.
What do YOU use?
As always, it depends on the context. You (the guy) could reach farther up her leg, yet you'd be hoping to further excite her. Yet you wouldn't go 'further' up her leg, since further isn't a physical measurement. Instead it's an indication of something not physical. So you could push yourself further to run an extra half-mile, yet the idea is to push yourself beyond what you'd normally be able to accomplish, NOT to simply run a specific distance. So, in that case, you'd say "Taylor pressed himself further than ever by running farther and crossing the final half-mile marker, his person record."
So, again, farther refers to physical distances while further refers to psychological of other non-physical distances. Still, if you even try to use both, you'll likely confused ALL of your readers. So as they say, pick one lane and stick to it, rather than continually switching back and forth and going nowhere any sooner while pissing everyone off!
Per the Chicago Manual of Style, 'farther' and 'further' have been used more or less interchangeably throughout most of their history.
Insisting on strict differences is akin to insisting never to split infinitives or never end a sentence with a preposition. Both are modern 'rules' which tried to impose external controls on a language that is decidedly uncontrolled.
There is no definitive answer for English word usage or grammar akin to the Académie Française. All that matters is that you are understood.
Per the Chicago Manual of Style, 'farther' and 'further' have been used more or less interchangeably throughout most of their history.
My 1999 COD entry for 'farther' directs to 'further'. In my part of the UK, 'farther' was never used and it looks slightly jarring whenever I see it, but I'm slowly becoming desensitised by USAian authored stories, where it seems to predominate.
The less common derivatives seem to be based on 'further' rather than 'farther' eg 'furthermost', 'furtherance'.
ETA From near parity around 1800, 'farther' has been declining in both British and American English Ngram corpuses, almost to the point of extinction in British English with American English not much better.
AJ
Wouldn't that be "furtheir"? or alternatively, simply "furrier"? (Historically the term applied to anyone who would skin and mount animals for display, yet could also apply in this context too. However there seems to be a discrepancy between using one 'r' or two (yet another British vs. American usage pattern?).