Think of an inciting incident as a kind of jumping off point for a narrative's plot. If you can identify the one moment, early on, that all the subsequent action stems from, you've found the inciting incident.
In the case of the play Proof by David Auburn, arguments could be made for several different moments being the inciting incident. One might argue that the inciting incident is the death of Catherine's father, Robert, which occurred before the play begins and is revealed by Catherine's vision of him in act 1, scene 1. Certainly, his death shakes his daughters to their core, causes Catherine's sister Claire to come to town, exacerbates Catherine's doubts about her own mental stability, and leads to the discovery of Robert's brilliant mathematical proof, in that it allows those left behind to search through his things for signs of genius among the rambling.
There is also an argument to be made that the inciting incident is when Hal reveals to Catherine that he's been going through her...
(The entire section contains 2 answers and 591 words.)