May I ask what this means: "I didn't know but what there was one gnawing at Grant?" When I look up in the Oxford Dictionaries, "not but what" means "nevertheless". Is the phrase related to the meaning which the Oxford dictionary describes?
Washburne referred himto J.Rusell Jones, a close friend of Grant go Grant and his investment adviser, who brought to the White House Grant's letter pledging that nothing could persuade him to be a candidate for President, particularly since there was the possibility of reelecting Lincon. "You will never know how gratifying that is to me," the President said after reading the letter. "No man knows,when that presidential grub gets to gnawing at him, just how deep it will get until he has tried it; and I didn't know but what there was one gnawing at Grant."