During a professional baseball game, a long fly ball is hit down the first base line toward the foul ball pole. if it goes to the right of the pole, it will be a foul ball, and if it goes to the left, it will be a home run. the umpire making this call will have the most difficulty making the judgment because .

Well, because it'll be very difficult to tell apart one object from another when one perceives there is an overlap between the two.

Explanation:

Let's remeber what Helmholtz said about perception, that it some times results from the assumptions we make about the enviroment without even being truely aware of it.

In this case, the umpire will have a really hard time judging wether it was "foul ball" or a "home run", given that his emotional status may pull him toward one call or the other.

Thankfuly, there are video-recording repetitions in baseball now.