In a strange and beautiful new play by Will Arbery, finding happiness is a process of failing upward.The difference between comedy and tragedy is often just a matter of timing. Bring the curtain down early enough and even “Macbeth” can have a happy ending; in the back story of a play full of laughs, you’ll often find a bucket of tears.Will Arbery’s “Corsicana,” which opened on Wednesday at Playwrights Horizons, is that second kind of play; if its story began any earlier than it does, it would be an emotional blood bath. Instead, without ignoring the bone-deep sadness of characters confused and stymied by loss, it lets us watch them climb their way out of it — heading toward joy and sharing some in the process.The immediate cause of the sadness for Christopher (Will Dagger) and Ginny (Jamie Brewer) is the death of their mother several months before the action. Though they have different fathers, both of whom have long since skedaddled, the half-siblings have similar reactions, within the framework of their evident differences.Christopher, 33, is a wannabe filmmaker who used to teach at a college near Dallas. He has now retreated to the melancholy comfort of his mother’s home, in Corsicana, an hour south. He’s done so, supposedly, to care for Ginny, 34, who has Down syndrome but doesn’t want to be babied. She’s a “grown woman,” as she is constantly forced to remind everyone. Yet she, too, has retreated: No longer volunteering at a nursing home, she instead spends most of her time watching Disney videos and listening to girl-power pop.“I can’t find my heart,” she tells Christopher, who likewise seems to have misplaced his. But if he is clueless about his own suffering, despite the torrents of words pouring out of him, he loves his sister too much not to act. He tries to help her re-engage with the world.How he does so, and how she responds, form the core of a play that is, paradoxically, almost too specific to describe. Weird, perhaps: Some of the characters are ghosts; there are longish passages of improvised song. Dense, certainly: It has the fuzzy texture of lived experience rather than the silkiness of honed argument. Quiet, mostly: The characters — also including a family friend named Justice (Deirdre O’Connell) and a hermitlike artist named Lot (Harold Surratt) — are the opposite of aggressive. In the face of their own deepest hopes, they are passive to a fault.Justice (Deirdre O’Connell) with Ginny, who has uncanny emotional intelligence — something her brother completely lacks. Sara Krulwich/The New York TimesFor those who loved the slashing debate and emotional frenzy of Arbery’s “Heroes of the Fourth Turning,” which ran at Playwrights in 2019 and was a finalist for the Pulitzer Prize, “Corsicana” will thus seem like an about-face. Directed with delicacy and patience by Sam Gold, it steers away from political discourse. Though Justice is writing a treatise on capital, we never hear a word of it; when ideology is discussed it sounds like sharing recipes.The play is nevertheless political, inseparably from its plot. Justice believes that Ginny, who likes to sing, might find something in common with Lot, who aside from making sculpture from trash writes songs from his spontaneous thoughts. But when Christopher approaches him to broker a deal that falls somewhere between babysitting and musical mentorship, it does not go well. He finds a man whose exclusion from society, partly self-imposed and partly not, have made him as forbidding as his (unseen) artworks, which Ginny, when she visits, calls monsters.“Corsicana” sometimes veers too close to the idea that the woman with Down syndrome and the emotionally troubled artist are magic touchstones, with deeper wisdom than others and purer ideals. Ginny has uncanny emotional intelligence, something her brother completely lacks. And unlike Justice, who has ulterior motives, Lot neither shows his work nor seeks to sell it: “Anything I make,” he says, “is a one-way street to God.”But before such moments can cloy, Arbery usefully complicates his case. When crossed, Ginny flounces and says inappropriate things; when upset, Lot goes rigid and sputters and spits. That Ginny very much wants a boyfriend with whom to experience adult pleasure is seen as natural and even wholesome but not without complications. Her erratic path toward happiness, sometimes causing collateral damage, looks a lot like Justice’s. And Christopher’s difficulty integrating a traumatic past into a productive present looks a lot like Lot’s.With so much going on, you can’t say that “Corsicana” — named not for a person or an idea, but a town — has a point. Instead, insofar as it’s a fully imagined world, it has hundreds. (Arbery calls it “an accumulation.”) Watching it, I felt it was about who gets to make art, and for whom. Reading it, I felt it was about how becoming “grown” is, for anyone, a lifelong process of failing upward. Thinking back on it, I feel it was about the way the world tucks beauty inside envelopes of sorrow, and vice versa.And yet I discerned, at an almost cellular level, a particular intention: to show that we all have an equal claim on happiness, if only we know how to stake it. To the extent that the play is autobiographical — Arbery’s sister Julia has Down syndrome — this is no doubt an expression of love. But it is also an effect of Gold’s direction, which feels communal, often placing actors in corners of scenes they aren’t otherwise part of. Even the set, by Laura Jellinek and Cate McCrea, cooperates: two identical living rooms coexisting under one roof.Though I was very moved by all of this, I understand why some theatergoers left at intermission the evening I saw it. At 2 hours and 30 minutes, the play can sometimes seem indulgent; parts of the story feel undigested and perhaps indigestible.Still, Gold and the actors have evidently made sense of it all, which was good enough for me. Brewer, who, like her character, has Down syndrome, is touching and hilarious in a fully realized performance. Surratt, neither caricaturing nor condescending to Lot, is astonishing. And even when Arbery gives Christopher an immensely long aria of self-discovery, and Justice what amounts to a mad scene (if love is madness), Dagger and O’Connell, who is fresh off a Tony Award for “Dana H.,” make it seem like falling off a building headfirst.Or really, heartfirst. Arbery seems to have written “Corsicana” with his internal censors set to their lowest setting, as if he were hoping to make music the way his characters do: for themselves and, as Ginny puts it, “with the door closed.” The tune may be strange and leggy and long, and you have no idea whether it’s funny or sad, but it feels like happiness to overhear it.CorsicanaThrough July 10 at Playwrights Horizons, Manhattan; playwrightshorizons.org. Running time: 2 hours 30 minutes. More