My point was that just because something can be defined as merely the absence of something else, doesn’t make the thing you are defining any less real. Since, as the quote from Isaiah that posted demonstrates, God has taken credit for not only creating light, but also darkness, even if it is only the absence of light. He created the phenomena of the presence as well as the phenomena of the absence, and both are equally “real” phenomena, no matter how you define them.
You are right that the boy’s argument doesn’t really rebut the professor’s assertion. Even if evil is only the absence of God from our hearts, an omniscient God must have known that condition would arise if he created beings with the freedom to choose to turn away from him. So even if the boy can successfully argue that God didn’t created the absence, the professor could reply that God created the opportunity for the absence, knowing full well that the absence would necessarily occur.
“My point was that just because something can be defined as merely the absence of something else, doesn’t make the thing you are defining any less real.”
Actually, it does.