Their View | Old rockers, new math: When is Yes a no?

“Who’s left from the original lineup?”

I posed the question to my longtime concert-going buddy when I heard the opening chords to “China Grove” in the midst of an encore. Onstage at Philadelphia’s Tower Theatre two weeks ago, with a banner reading “The Doobie Brothers” behind them, were a half-dozen musicians who looked like a mirror image of much of the crowd, including us.

“Two,” replied Paul Lauricella, without need for deliberation, naming Tom Johnston and Patrick Simmons. He might be a trial lawyer by day, but ensconced in a rock hall, Paul is a walking encyclopedia.

The Doobies were opening for Peter Frampton, who I’ve pretty much been seeing annually since the summer after ninth grade. That year, 1977, I was among 91,000 others at JFK Stadium for a concert advertised as “High Noon,” with Frampton headlining a bill that included Lynyrd Skynyrd and the J. Geils Band.

During the intermission, Paul and I perused the merchandise table in the Tower lobby, which included a Doobie Brothers multi-CD set called the “Farewell Tour” — as in, the 1983 farewell tour. Nearby were fliers advertising this summer’s return of Yes, which will perform the albums “Fragile” and “Close to the Edge” in their entirety on July 19. Also on the wall was an ad for Styx, Foreigner and Don Felder (“formerly of the Eagles”) at the Susquehanna Bank Center in Camden, N.J.

This incarnation of Yes lacks legendary frontman Jon Anderson, and I told Paul that I had just seen an ad for a show noting that Dennis DeYoung will be performing “The Music of Styx.” Paul noted this version of Foreigner also lacks its original frontman, Lou Gramm, who left in 2004.

We debated the propriety of aging musicians using the original band name when there are so few original members in the lineup.

I proposed a two-fifths rule, but Paul waved me off.

“You can’t do it with a simple mathematical analysis. You could just as easily say it’s enough if at least half the band’s members were with the original incarnation. But by that measure, Paul and Ringo can constitute a reunited Beatles, and those two guys currently calling themselves The Who would actually be The Who.”

He then promptly dismissed the three-fifths compromise I offered.

“By that count, the current iteration of Creedence Clearwater Revival would be the genuine article, notwithstanding the conspicuous absence of John Fogerty — arguably the voice, the sound and the spirit of the group.”

So it’s the quality of the personnel that counts, not the quantity. Consider that the current touring version of Cheap Trick wants you to want them despite the absence of Bun E. Carlos, but you get vocalist Robin Zander, cartoon-genius guitarist Rick Neilsen and bass impresario Tom Petersson. So for the band that brought us “Live at Budokan,” three-fourths of the group is enough to warrant a claim to authenticity based on the quality, not the 75 percent representation.

Even ’90s grunge kings, Stone Temple Pilots, recently toured without frontman Scott Weiland. STP without Weiland is like Tom Petty and the Heartbreakers without Tom Petty. Queen is another band touring with a new lead singer. Notwithstanding its likely spectacle, the Adam Lambert-fronted Queen is simply not Queen.

Good news is that this summer will also find Aerosmith on the road sporting the original Boston-ians, including Steven Tyler and Joe Perry. The Eagles are doing a chronology tour with four-fifths of their classic lineup, while Rod Stewart, Tom Petty and Paul McCartney will all be playing themselves at an arena near you.

At 72, Sir Paul may only be 80 percent of the rocker he once was, but four-fifths of Paul McCartney still constitutes 20 percent of the Beatles, and that’s all right.