You (my husband) say farther and I say further. Not only that but we are both sure we’re right. How can this be? To the benighted farther brigade it is obvious. Farther is the comparative of far, so, at least in the literal sense of distance, it is the logical form. Such instincts to tidy up language are natural.
Indeed a previous comparative was farrer, very logically. This held sway from the 12th to the 17th century, after which it began to be associated with the sort of speech heard on The Archers. (Originally the comparative of far was fyrr, but that was before the Conquest.) The forms farther, further, which came to supplant farrer, were modelled on the noun further, in the meaning of ‘furtherance’.
So much for history. How should anyone decide now whether to use further or farther? Dickens, for example, or his printer, uses either form before the noun end (of a room). The Oxford English Dictionary says: ‘In standard English the form farther is usually preferred where the word is intended to be the comparative of far, while further is used where the notion of far is altogether absent; there is a large intermediate class of instances in which the choice between the two forms is arbitrary.’
That would seem to be that — except this judgment was not formed in 1989, when the second edition of the OED was published, but in 1895, when words beginning with F first came out. Even by 1926, when the stickler Henry Fowler published Modern English Usage, it was true that ‘hardly anyone uses the two words for different occasions; most people prefer one or the other for all purposes, and the preference of the majority is for further’.