Take a look, perhaps walk around a bit. Are there windows or doors with glass panes? Check them out. What is it like to look through them? How is the view? The cleaner the glass, the less of it you see – and the better your view through it, right?
People wash and clean their windows (and mirrors), to improve their view. Blemishes distract from what you want to see. As a medium that glass should accommodate a view, but should itself disappear – and not distract – from it.
The effect of that can be convincing indeed. Perhaps you have bumped your head or hand against the glass, when you lost sight of the fact that there was in fact something between you and what you were looking at.
You are probably familiar with the experience of wearing glasses. Perhaps you do not wear them generally or not even just to read, but chances are very good that you have worn sunglasses one time or another, when out and about.
Glasses, specifically their lenses, can get dirty, easily. You notice, when there is grease or bits of dust and you take them off to clean them, so that you can see better. When the lens is very clean, your view through it is very good, but your view of the lens itself is gone.
If this has not yet happened to you, then sooner or later it will: you are doing your best to navigate to a location – maybe you are visiting a different city or are just in an unfamiliar neighborhood – and despite your best efforts you end up stuck. The map you use eventually led you astray.
Perhaps that map is very outdated or there is a complicated, construction-related detour or you fell victim to a trap street (an intentional map error). Either way, you are temporarily stuck and need to problem-solve your way out of this and to your eventual destination.
There is a good chance you can look around, orient yourself and tell, where you are in your environment. At that point you realize that the map – though looking clear and likely precise – is clearly mistaken.
One way or another, you are invariably looking through lenses. The clearer the view, the easier that is to forget. Smudges on the pane are easily seen as annoyances to fix, but they also serve as an important reminder.
You are looking at a map, not the territory.