"No, he couldn't have done."
"Why not?"
"Because it would have been impossible."
"But why would it have been impossible?"
"Because he couldn't have done it."
"I suggest that Mr. Bellingham left the house quietly while you were on the stairs?"
"No, he didn't."
"How do you know he did not?"
"I am quite sure he didn't."
"What makes you feel sure he did not?"