The moment I heard of a fire being lighted in Mrs. Inglethorp's room, I leaped to the conclusion that it was to destroy some important document ”possibly a will. 
I believe that Mr. Inglethorp's will was destroyed. 
mnli hypothesis: I believe that Mr. Inglethorp's will was destroyed.  premise: The moment I heard of a fire being lighted in Mrs. Inglethorp's room, I leaped to the conclusion that it was to destroy some important document ”possibly a will. 