That thing, apparently, is Wilson. House goes to his apartment and says he needs an epiphany and he'll even pay one hundred dollars more than Wilson's billing rate to get it. Wilson doesn't say anything, so House crumples up the money and tosses it in his apartment. Which, by the way, was CTB's apartment. House admits that Wilson's "sloppy" and "undisciplined" thought process contrasts nicely with House's and causes him to think of things he normally would not. While House says this, he looks everywhere but at Wilson. Wilson tells House to go away. House ignores him and starts saying he's got a patient who has "cancer, but not cancer." Wilson tries to close the door, but House blocks it. He sighs and asks Wilson how he's doing. "Don't do this. Please. Please, don't do this," Wilson says; "I'm trying to move on." And then House does something stupid and gets angry. He asks Wilson if talking to Cameron, Cuddy, Foreman and everyone but House is part of the moving on process. Wilson looks at him like, "how and why do you know that?" House realizes he's caught and admits to hiring a PI to spy on Wilson. That's just scary. House justifies this by saying that Wilson has to talk to House and "deal" with him if he wants to move on, as opposed to having nothing to do with him, which is how most of us move on.
Wilson says House has no right to do this. House says if they're not friends anymore, then he has the right to do whatever he wants. There's no trust between them to breach. House can tell everyone Wilson's secrets and have him followed. Oh my god, House is such a child. A child! I think I pulled something like this (in terms of telling an ex-friend's secrets, not the PI part. But only because my allowance wouldn't cover that) when I was eight. With that, House starts talking about his patient again like nothing happened. Wilson says he has rights, too. He has the right to walk away from House and live in the wide world that exists beyond House. And House should realize that world exists, too. "The next time you knock, I'm not answering," Wilson says. "Nothing yet. Keep talking," House says. This is definitely the way to rescue a faltering friendship. And by that I mean getting "the door slammed in your face."
A defeated but not ready to admit it House limps down the sidewalk past the PI. "I'm sorry," he says, having listened to the conversation that just transpired. House asks the PI how many friends he has. "Seventeen!" the PI answers immediately. He doesn't really have seventeen friends; he just pulled out a number because he knew House was only asking the PI the question to refer back to himself. "I have one," House says; "had one." What about Cuddy? Why aren't they friends? The PI says friends are important, and House asks if he's charging him for this. The PI says he is if they aren't friends, but when House asks if they can be friends (because he's cheap) the PI declines, saying House scares him. House doesn't understand why Wilson has to be his friend for them to talk. At this, the PI says something that makes no sense but gives House the epiphany he needs: "friends are friends. Customers are customers. And everything else is everything else. If it's not, nothing is nothing."