Nostalgia is no science, it’s a sentiment, and a tricky one at that. It’s a response to something sensory: a fragrance, a sight or a sound. Few stimuli evoke nostalgia like a song, especially one that stirs warm but vivid, far-away memories.

On paper, Friday night’s show before more than 2,500 fans at Starlight Theater was all about the past and the place of songs in people’s lives. In the end, it proved that there’s more to nostalgia than merely recalling something.

The co-headliners were Boz Scaggs and Michael McDonald. Scaggs’ career goes back more than 40 years and a debut album that included studio help from Duane Allman. But he is best known by far for his “Silk Degrees” album, released in 1976, which produced three Top 40 singles, one of which hit No. 3, “Lowdown.”

His co-headliner was Michael McDonald, 59, former Doobie Brother and multi-Grammy winner. McDonald opened a show that lasted three hours, including a 20-minute break, and comprised more than two dozen songs.

“You Belong to Me,” a song he co-wrote with Carly Simon (and which she made more famous), opened his set. He followed that with a Doobie’s standard, “It Keeps You Running.”

McDonald spent his set seated behind his keyboards, before the seven-piece backup ensemble he shared with Scaggs. His voice still carries plenty of its signature gritty white soul, but this evening, it seemed to be lacking slightly in strength, especially in his upper register.

Right away, his performance set the tone for the evening. The show was like a still life in soft rock. Neither he nor his band showed much animation throughout the set. Whatever personality came off the stage was courtesy of Miss Monet, one of the two backup singers.

The fine-tuned five-piece band didn’t do much more than stand and deliver its duties. Its solos were relatively straight-up and standard: high-end chops in the rock-blues form.

McDonald would deliver his best-known songs from the Doobies’ catalog — “Minute by Minute,” “What a Fool Believes” and “Takin’ it To the Streets ” — and the Grammy-winning song he co-wrote with James Ingram, “Ya Mo Be There.”

But he also played several well-known covers, including three Motown solid-gold nuggets: “I Heard it Through the Grapevine” and “Ain’t Nothing Like the Real Thing” and “Ain’t No Mountain High Enough.” None of his versions did anything daring or surprising to the original. Subsequently, it felt like we were listening to a world-class wedding/cover band. Except no one was dancing. In fact, most of the night the crowd was in repose, as if watching a recital.

Scaggs’ got off to an enthusiastic start with two from “Silk Degrees”: “What Can I Say” and “Georgia.” He played two more from that record, the slinky, funky “Lowdown” and then the poppy “Lido Shuffle.” Both started some dancing, in the seats and in the aisles.

Scaggs’ voice these days doesn’t sound all that removed from the “Silk Degrees” era, though his material doesn’t require the visceral soul that McDonald’s does.

His set got a bit bogged down a couple of times when he sang newer material (“Some Change,” “Desire”). He, too, led the band through a couple of covers: of Fats Domino’s “Sick and Tired”; and of Bonnie Raitt’s “Let’s Give Them Something to Talk About,” with help from Monet.

He ended with two of his better-known songs, “Look What You’ve Done to Me,” the love ballad from the “Urban Cowboy” soundtrack, and then “Lido Suffle.”

For the encore, he and McDonald came out to perform four more covers. Lots of people have taken on Leonard Cohen’s “Hallelujah” with varying degrees of success and necessity. This keyboard/guitar version — a stark gospel/soul rendition — was a bit too self-indulgent for my tastes. It overwhelmed the hymn’s lovely melody and became more about the vocals than the lyrics.

They ended with three more covers: Joe Simon’s “Drowning in the Sea of Love,” which was part of last year’s “Rock Soul Revue” featuring McDonald, Scaggs and Donald Fagen; Chuck Berry’s “You Never Can Tell”; and Curtis Mayfield’s “It’s Allright.”

Each was done proficiently and with the right dose of sincerity, polish and precision, but none evoked anything more nostalgic or than the sense that these songs belong to other famous people and to another era.

Michael McDonald setlist: You Belong to Me; It Keeps You Runnin’; Sweet Freedom; Here to Love You; I Keep Forgettin’; I Heard It Through the Grapevine; Stop, Look and Listen; No Love to Be Found; Yah Mo B There; Minute by Minute; Ain’t No Mountain High Enough; Ain’t Nothin’ Like the Real Thing; What a Fool Believes; Takin’ it to the Streets.

Boz Scaggs selist: What Can I Say; Georgia; Some Change; Desire; Sick and Tired; Breakdown Dead Ahead; Lowdown; Miss Sun; Let’s Give Them Something to Talk About; Look What You’ve Done to Me; Lido Shuffle.

Encore (together): Hallelujah; Drowning in the Sea of Love; You Never Can Tell; It’s Alright.