The human emotional response presents an apparent paradox.
Principle
People believe that they can be genuinely moved only by those things and events that they believe to be actual, yet they have genuine emotional responses to what they know to be fictional.
Evaluate / Goal
Usually principles are conditional rules, and so we could latch onto the "only" part and write half of the 'apparent paradox' as a conditional rule:
If X is not something → ppl don't believe that X ppl believe to be actual can genuinely move them
But it seems like this principle is intended to stress both halves of the paradox. We think we can only be genuinely emotionally triggered by real stuff, but in reality we have genuine emotional responses to fictional stuff.