In the strictest sense, use farther for physical distances and further for figurative distances. But Eric Partridge and Bryan A. Garner both agree that further, which has always been the more common of the two words, can serve for both physical and figurative distances. Do not, however, apply the same logic to farther. Both The city is farther away and The city is further away are acceptable, but The problem requires farther thought is not. Their superlatives, farthest and furthest, follow the same convention.