Hello, Toni - and welcome to English Forums. In grammar books, 'further' is used for extent or mental distance ('He writes further that man is just an animal') while 'farther' is used for physical distance ('My arm can reach farther than yours'). In real life, 'further' is taking over for 'farther' for physical distance and in another 200 years or so, maybe 'farther' will have become archaic.
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