(FORTUNE Magazine) – It's a paradox: John Walton's contributions to Wal-Mart would be difficult to calculate, but at the same time he will be almost impossible to replace. The 58-year-old son of Sam Walton died after an experimental aircraft he was piloting crashed near the Jackson Hole, Wyo., airport on June 27. John never held any title at Wal-Mart (except, at one time, company pilot). But when his father asked him in the early '90s to serve on the board, John jumped at the chance, telling me in an interview last year that he was "honored" by the opportunity. Board members say John was noted for his clearheadedness and moral compass. "He brought a different perspective to the board," says Wal-Mart CEO Lee Scott. "He wanted to increase our focus on the environment, education, and literacy. Our reputation too. 'We can't just brush that off,' John would say." That type of non-insular thinking by an insider is critically important to the retailing giant these days. In addition to his role as a board member, John also took the lead in his family's philanthropic efforts, pushing the Waltons to donate hundreds of millions of dollars to educational projects.

John Walton was memorialized on June 30 at the small Presbyterian church he attended in Jackson Hole. The mourners included his three siblings and his mother, Helen, who constitute America's richest family, with a fortune of some $100 billion. (It's not known whether John's wife and son will sell any of his $19 billion of Wal-Mart stock or whether there'll be any change in the family's stake in the company.) Also attending the funeral were Wal-Mart CEO Scott and his predecessor David Glass, as well as business leaders such as Ted Turner. They remembered Walton as a hero for his harrowing missions as a Special Forces operative in Vietnam and for working tirelessly for education reform. Not everyone agreed with John's positions on school choice, but no one doubted his desire to help children receive a better education. John was also eulogized as a lover of nature, an entrepreneur (he ran a boat-building business and owned a solar-power company), and a man who was effortlessly genuine.

Last fall I spent a day driving around the delta region of Mississippi and Arkansas with John, visiting schools he was funding. John was no extrovert--he was kind of the strong, silent type--and getting him to open up was challenging. He answered many of my questions with just a polite word or two, his face mostly expressionless. That all changed when we arrived at a school and John met the students. With them, he made small talk and shook hands, his face breaking into a wide grin. Later, near the end of an interview in an empty classroom, I asked what he would have done with his life if he hadn't been the son of Sam Walton and co-owner of 39% of Wal-Mart. "I have no idea," John said. "What is, is. I'm much more concerned about where I'm going tomorrow than what I would have done." Sadly, we are now left wondering where John Walton would have gone tomorrow, as well as what else he would have done. -- Andy Serwer