At the start of The Simpsons' second season, Fox's upstart animated hit was already a cultural phenomenon, but not quite a critical one. The Bart Simpson t-shirts were flying off the shelves, but the show's initial success was still new enough to dismiss it as a fad. It's impossible to pinpoint exactly when The Simpsons took root as the definitive television show of the 90s (and probably the best sitcom ever made), but a crucially underrated moment is "Treehouse of Horror," its first Halloween special and the third episode of season two, which aired on October 25, 1990. Perhaps more than any other, it's an episode where The Simpsons' co-creator Sam Simon's massive effect on the show's intelligent, formula-shredding identity was clear.

Simon died of colon cancer on March 8 at age 59. In the years following his 2012 diagnosis, he was noted for defying the three to six months he was given to live and for donating most of his massive fortune to charity, with a particular focus on animal rights.

Most Americans who grew up watching The Simpsons have seen his name in front of every episode: While Matt Groening created the show, Simon helped the cartoonist develop it for television alongside James L. Brooks and worked on its first four seasons, lending it an experienced TV showrunner's hand (he had worked on Taxi, Cheers and It's Garry Shandling's Show, among others). Supposedly, he and Groening never quite creatively gelled, and Simon departed with a lucrative payoff, having helped lay the groundwork for a show that entered its 26th season last year.

While the behind-the-scenes details remain cloudy, it's clear that Simon established many of the core principles that helped The Simpsons stand out from the sitcom pack and set the bar for intelligent animated programming on primetime TV, a tradition especially encouraged by Fox to this day. Simon had the voice-over cast perform together to encourage comic snap and emotional realism; he brought in some of the show's most enduring writers, like John Swartzwelder, Jon Vitti, and Al Jean. When season two rolled around, The Simpsons was moved to a competitive Thursday night slot against NBC's The Cosby Show, a sign of Fox's faith in the program as the anchor of its lineup. The season’s first episode, “Bart Gets an F,” got the highest ratings in its history, but “Treehouse of Horror” was the real curveball: a genre-busting effort that proudly punched above the show’s expectations.

Watch it today, and "Treehouse of Horror" seems like a lot of early Simpsons: funny and smart, but rough around the edges, from the animation to the voice acting to the pacing of the humor. That may be one reason Simon isn't so frequently cited in discussions about the show's legacy—he left just as it was entering its truly bulletproof years (generally accepted as running from around seasons four to eight). The first Halloween special has three short segments, an annual tradition the show honors to this day. In the first, written by Swartzwelder, the Simpsons occupy a haunted house, which they eventually drive insane; the second, written by Jay Kogen and Wallace Wolodarsky, is a Twilight Zone spoof about people-eating aliens.

The last segment is credited to Edgar Allen Poe and Sam Simon. It is a direct and very faithful retelling of "The Raven," starring Homer as its lead character and his son Bart as the tormenting raven, narrated by James Earl Jones. Poe's flowery, musical language is mostly retained, and there are basically no jokes. It's an unusual moment compared to most of The Simpsons' output, but it's an inarguably effective retelling, somehow beautifully compressed into only a few minutes, the kind of thing the entirety of the show's broad audience (from the kids wearing the Bart Simpson T-shirts to the parents who bought them) could appreciate.

Fox/The Atlantic

Groening noted on the DVD commentary just how worried he was about "The Raven" coming off as pretentious, but Simon (and director David Silverman) nailed the task. The episode is scary, but all thanks to the uniquely weird atmosphere it manages to generate despite telling such a familiar tale. It’s funny, but mostly because of its committed voice acting—it doesn’t rely on cheap meta-gags to acknowledge its unusual format. It marked a graduation moment for The Simpsons, one that helped prove it was no crass cartoon for teenagers reliant on physical humor and recycled catchphrases. The show would prove that point over and over again, and of course Simon was just one of many factors in that ongoing triumph. But he was an indispensable element—as Vitti told The New York Times years ago, "If you leave out Sam Simon … you're telling the managed version. He was the guy we wrote for."

The sum of Simon's career is more than just The Simpsons, and his efforts post-retirement to leave behind a truly humanitarian legacy were an unmitigated victory. But even forgetting that, the cultural legacy of The Simpsons cannot be understated. If he had made his mark nowhere else, Simon's work in helping to usher in a new era of television comedy as an art form is one of the medium's great feats.