Phil Lesh, co-founder and bassist of the Grateful Dead, died on Friday at the age of 84.

Lesh's death was announced on social media with a short statement: “Phil Lesh, bassist and founding member of the Grateful Dead, passed away peacefully this morning. He was surrounded by his family and full of love. Phil brought great joy to everyone around him and leaves a legacy full of music and love. We ask that you respect the Lesh family's privacy at this time.” A cause of death was not given.

Since the Dead's earliest incarnation as the Warlocks, Lesh enjoyed a three-decade-long close partnership with lead guitarist Jerry Garcia. He also took responsibility for their long improvisatory tendencies, electronic experiments and nightly free-form “space” interludes. After the group disbanded in 1995 due to Garcia's death, Lesh became an active keeper of their live flame, both in various configurations with former band members and in several iterations of Phil Lesh and Friends, which included numerous guests from the extended, cross-generational improvisation -Rock community.

Philip Chapman Lesh, the Dead's senior member, was born on March 15, 1940 in Berkeley, California. His father repaired office machines and his parents owned a repair business together. Lesh played viola and trumpet at school, but gradually became more interested in composing than performing. He attended UC-Berkeley, where he befriended the even more musically adventurous Tom Constanten (who played keyboards with The Dead for a time), but dropped out in the first semester. With Constanten he attended a course taught by the great Italian avant-garde composer Luciano Berio at Mills College, where he met the future poster boy of minimalism, Steve Reich, with whom he collaborated on a musical “happening” called Event III/Coffee Break.

In 1959, Lesh met Garcia at a house party in the Bay Area, to which he had been directed “as if by an invisible hand,” according to his 2005 memoir. When he met Lesh again after a Warlocks performance in 1964, Garcia invited him to join the band on bass guitar, an instrument Lesh had never played. Lesh played his first show with the Warlocks the following year at Bikini A-Go-Go in Hayward, California. The renamed Grateful Dead became the house band for Ken Kesey's infamous Acid Tests. Lesh, a staunch advocate of psychedelics as evidence of “a spiritual realm,” was deeply affected by these evenings, which blurred the line between band and audience.

The Grateful Dead played “electric chamber music,” according to Lesh, whose main influence as bassist was Johann Sebastian Bach's style of counterpoint (the relationship of two independent but interdependent musical voices). When he wasn't dropping his infamous “bass bombs,” he played his instrument like a low guitar, usually with a pick and often like a lead instrument. The 1960s became an era of intense musical experimentation for the group, particularly on the band's second album. Hymn of the Sunwhere Lesh suggested overdubbing several different live versions of “The Other One” on top of each other and allowing them to drift apart. “I have nostalgic feelings for the Psychedelic Ranger era when we played anthem live in its entirety,” he told Rolling Stone in 2014. “It was apocalyptic every time.”

“Box of Rain” and “Unbroken Chain” were warm psychedelic masterpieces among the few songs Lesh co-wrote for the Dead. He contributed the high parts to the four-part harmonies that the band mastered Workingman's Dead And American BeautyHowever, he eventually left the singing to others, although the audience occasionally shouted “Let Phil sing!” In 1975 he played electronically processed bass on electronic musician Ned Lagin's Abstract Sea stones.

Although touring had lost its appeal, Lesh continued to struggle with the Dead through the increasingly difficult '80s, as drug problems rocked the band, then into the '90s, which ended with Garcia's death. “Jerry was the focal point,” he said Rolling Stone. “We were the spokes. And the music was the profile on the bike.”

In 1998, Lesh received a liver transplant for hepatitis C, which he had contracted decades earlier. The procedure led to him becoming a passionate advocate for organ donation. He survived prostate cancer in 2006.

A one-off acoustic show in 1994 featuring some members of the Grateful Dead was billed as “Phil Lesh and Friends”, a nickname he would use for performances with an ever-changing musical lineup for the rest of his career. The group's post-Dead debut in 1999 featured Phish's Trey Anastasio and Page McConnell. Lesh also released three jammy rock albums under this name. During the Aughts, Lesh sometimes joined his former bandmates in such Grateful Dead repertory formations as The Other Ones, The Dead and Furthur, which also featured Bob Weir. In 2005, Lesh published his memoirs In Search of the Sound: My Life with the Grateful Dead.

Lesh's wife, Jill, whom he married in 1984, became a close partner in all aspects of his life. In 2012, the Leshes opened the restaurant and event venue Terrapin Crossroads in San Rafael, California. Their sons Grahame and Brian serve as the house band, and the bassist himself has been known to sometimes accompany evenings of live Dead karaoke.

Lesh and the rest of the Dead celebrated their 50th birthday^Th anniversary in 2015 with a series of “Fare Thee Well” shows in Chicago, which Anastasio attended. This year, Lesh announced that he had bladder cancer. “I’m one of those guys who’s always open,” he said in 2013. “Look, music is infinite. There are infinite ways to do it, infinite melodies that can harmonize with a one-four-five progression, it's absolutely infinite, no floors, no ceiling.”