Mr. Inglethorp's reason for not returning last night was, I believe, that he had forgotten the latch-key. 
Mr. Inglethorp left his key at the restaurant and couldn't return.
mnli hypothesis: Mr. Inglethorp left his key at the restaurant and couldn't return. premise: Mr. Inglethorp's reason for not returning last night was, I believe, that he had forgotten the latch-key. 