I would never have dreamt of suspecting the doctor, had not Poirot put it into my head. 
Until Poirot put the idea out there, I never even suspected the doctor of doing such a thing.
mnli hypothesis: Until Poirot put the idea out there, I never even suspected the doctor of doing such a thing. premise: I would never have dreamt of suspecting the doctor, had not Poirot put it into my head. 