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