Szarkowski uses mirrors and windows ad a way to describe photography and the artist behind them. The use of mirrors reflects the feelings or mental state of a person while the window reflects an ambition or image that the artist would like to see.
Both pictures reflect subjects in similar positions with different meanings. Gary shows a window where the subject could be some he knows or is familiar with while Gregory reflects a set up scene where the subject might be having a bad night in many different ways. Creating both a mirror and a window takes the artist in different positions while themes or other factors they decide to photograph.