What we see

I've just been starting the exercise I like to try with first-year students each year, hoping to begin encouraging them to see more in the world and to recognise more ways of seeing the world, where I ask them to tell me what they see when they look out of the window. As the discussion has developed this time around, one of the students this week picked me up on my comment that "what I see depends only partly on what is actually there". Thinking about that, I started wondering what we would get if we tried to graph what we see against what is actually there. Plot a graph with "what is actually there" on the vertical axis and "what I can see" on the horizontal. For people who see exactly what is there, they will no doubt expect their line to be straight and sloping at 45 degrees. Did I just write "no doubt"? I must be tired.