He looked up at my window as he passed, and I recognized him, though he had changed much in the fifteen years that had elapsed since we last met. 
I noticed that he had not changed in fifteen years because I was engaged to him.  
mnli hypothesis: I noticed that he had not changed in fifteen years because I was engaged to him.   premise: He looked up at my window as he passed, and I recognized him, though he had changed much in the fifteen years that had elapsed since we last met. 