There are some people in your life who are like windows. You don't even see them, but they give shape and perspective to what you do see.
We look at windows when they are dirty or broken, or when we are thinking about building or replacing them.
People like windows are people whose point of view we trust so much that we stop thinking about it as a point of view and think of it as the way things are. Until the trust breaks - because we change or our window does - we believe what we see and what we see is shaped by the window through which we are seeing.
It's hard to see a window until it breaks. Look back, and you will find the people in your past who were windows for you - windows into how friendships work, or how things should be, or windows into a particular job or field. At the time, you thought they were the whole of the picture.
Look around you now. Maybe - in your peripheral vision - you will catch a glimpse of an edge and know that you are looking through a window.