A fear of drowning? It can be seen in the left hand corner, with the three horses: one head can be seen layn over the other’s back, as if he were tripping. The picture shows what happens BEFORE he fears the most: the horse trips, and the whole left carriage falls into the water, everybody drowns. It explains everything the professor said I think:
“Remove all objects”: Water is a liquid, so you can’t remove the water.
“One tiny thing”: Just the horse’s head.
“Find the phobia, find the answer”: Explained above.
“Fear of open spaces is close”: Water is everywhere. The person who made this has gone insane because he’s afraid of falling in the water, so he fears going outside as well, going insane slowly.
“Water and air”: He’s afraid of drowning, not being able to BREATHE.
The picture also may incur a fear of death itself, most likely by accident.