Because if Mr. Inglethorp knew that his wife would be poisoned last night, he would certainly have arranged to be away from the house. 
Mr. Inglethorp was in a different county that night his wife died.
mnli hypothesis: Mr. Inglethorp was in a different county that night his wife died. premise: Because if Mr. Inglethorp knew that his wife would be poisoned last night, he would certainly have arranged to be away from the house. 