Donovan and I are holding hands over coffee in the Glasgow sun.

We're sat so close our legs entwine as he sings me a song he once wrote about sunshine, and spins me winding, colourful yarns about post-war Maryhill, transcendental super-vision, Pink Floyd, Billy Connolly and how he influenced The Beatles.

The 1960s pop visionary blazed a trail for psychedelia, Celtic rock and flower power, and inspired bands from Led Zeppelin to Belle and Sebastian. Such righteous feats secured his position at the heart of a canon that sometimes forgets him. Perhaps this is why he's not slow in reminding us in whose company he belongs: "The heroic poets, the higher songwriters ... Joni Mitchell, Leonard Cohen, Bob Dylan, Donovan, Neil Young. I could go on," he congenially offers. And so he does.

An interview with Donovan is an audience with Donovan. And this audience with Donovan is intimate indeed.

Eight films that feature Scots singer Donovan's songs

We meet in a suite in One Devonshire Gardens, a stone's throw from Maryhill, where he was born Donovan Leitch in 1946. We sit on sofas across from each other, but he gradually comes around, and pulls an armchair right up beside me. He educates me in the meaning of bliss (via meditation), and rarely seems happier than when he's recalling the women from his infancy going dancing down the Barrowland. "They'd all be in furs - the mammy, the grannies, the aunties - and full of perfume, with that great red lipstick, and they'd lean down and kiss me as they left," he says, a glint in his eye. "That was OK you know, being surrounded by seven women all the time."

Women have had starring roles in Donovan's songs and mythology since those days of dolled-up, scent-billowing matriarchs. We've grown to know and love Jennifer Juniper, Guinevere, Lady of the Stars, Susan on the West Coast Waiting, Mellow Yellow's Saffron, and Legend of a Girl Child Linda, which is one of countless tributes to his enduring muse and partner, Linda Lawrence (when that song was written, she was still the girlfriend of the Rolling Stones' Brian Jones). The dedication in his 2005 autobiography, The Hurdy Gurdy Man, simply reads, "For She ..."

Donovan's patchouli-loaded fables follow suit. He frequently invokes what he calls "the power of the feminine" - from ancient tales of women fighters and prehistoric dominant spirits, to quoting Billie Holiday, saluting Beyonce and allying his Maryhill roots with Maggie Bell. "It's all about the goddess, Nicola," he nods sagely. He counts the chakras on my spine. He shares my cup and eats my biscuit.

It is 50 years since Donovan released his debut single, Catch the Wind. The loved-up folk psalm debuted on the UK singles chart on March 31, 1965 (it peaked at number four) - the same week that Bob Dylan, with whom he'd often find himself compared, also made his chart debut with The Times They Are a-Changin' (it reached number nine). Half a century on, the erstwhile "British Dylan" is returning with a brand-new single, One English Summer, a hand-selected career retrospective and a Glasgow date which feels like a timely homecoming for the romantic outsider who has variously dwelt in London, Ireland and on higher planes.

Does he feel at home when he's back in Glasgow? "It's scary," Donovan replies. "Why scary? Well, because my memories of those first 10 years of my life when I lived here, in Maryhill and then in St Vincent Street - before we moved to Hertfordshire - were always dark and grey.

"It was granite stairs and the mammy washing them. It was me getting the polio when I was five. And it was after the war, so the city was bombed out - all the buildings, or a lot of them."

It's a curious tale, this story of a sickly boy from post-war Glasgow whose lexicon became uniquely gilded in amber, yellow and gold. Traditionally, Scottish pop artists tend to reflect our gloomier skies - The Blue Nile, Deacon Blue, The Waterboys, Frightened Rabbit (Sing the Greys), Belle and Sebastian (The Blues Are Still Blue) - but Donovan's work was always illuminated by the sun. And he, in turn, shone a light on life and love: Summer Day Reflection Song, Voyage Into the Golden Screen, Sun. You might wonder how such a bright idiom emerged from illness, rubble and darkness. You might even ask him. But Donovan is not one for direct answers.

He is, however, a consummate storyteller. So after deviations into the history of the British Isles, ages-old tribeswomen fostering offspring, and Glasgow's unbeatable knack for culture and shipbuilding, Donovan hits on an explanation as to why his music radiated brightness. Art was a beacon. "At first it was quite dark, when I started looking back on Glasgow," he explains. "But then I remembered the songs that the mammy sang, the aunties sang, the uncles sang. And then I realised that in all that darkness and oppression and poverty - so-called - it wasn't really dark at all. There was music. There was poetry. There were songs. And that was everything."

You can trace many of Donovan's touchstones - folk, jazz, poetry, bohemian romanticism - back to his Glasgow childhood. "Everybody had a way of singing in my family," he says. "And I don't just mean folk songs. Mammy sang Frank Sinatra, an auntie sang Nat King Cole. So at the party - in the front room, in St Vincent Street, three floors up, the tram cars coming by - a slightly tipsy relative would be forced on to a chair - 'Gies yer song!' - and all the wee boys and girls under the table, with the shandy, would listen.

"Nobody played a musical instrument, except my uncle Bill. He was a kind of bohemian. He played guitar. He died in a motorbike crash with a girl on the back. Many years later, Billy Connolly said to me, 'Your uncle was Postie!' - because he was a postman, and he was quite well-known in Glasgow. Only later did I think about it and realised: this was the guy who, when I was a kid, must have first sat me in front of a guitar."

And then there was Donovan's father. "He'd stand up in the middle of the room, and recite poetry for half an hour," he recalls. "And now we're talking about the bardic tradition, and that's why I am so powerful and skilled in my work - it's because he taught me, from the age of five. Some of the poetry he'd read would be bawdy. Sexy. Funny," he says with a laugh. "But other times, he'd read high poetry of noble thoughts. I think that's where I got the idea that we've been here before - that belief in the ancient Irish/Scottish tradition of reincarnation. And that's what eventually sent me to India. That's why The Beatles and I became friends."

Donovan and his enduring sidekick, Gypsy Dave, met The Beatles when they hitch-hiked from Hertfordshire to London in search of Bohemia. He later joined the Fab Four on their infamous 1968 trip to India, whereupon he taught Paul McCartney, John Lennon and George Harrison guitar finger picking techniques. Harrison was later quoted as saying, "Donovan is all over The White Album", and Donovan tells me a story about helping Lennon write the lyrics for Julia. But I'm slightly distracted because, as he relates this, he reaches across my lap, to my cup and saucer, helps himself to the shortbread that's on it, gesticulates with the biscuit briefly - the better to delineate Lennon's tragic genius - and then he eats it.

"You've got to understand," he continues, chewing, "we so-called spiritual songwriters of the 1960s were very well read. And why were we reading? To find out the answer to the question. When Gypsy Dave and I arrived in the pop community, we added something that they didn't actually have at the time. Now, that wasn't just how to make Sergeant Pepper, or how Pink Floyd would make Dark Side of the Moon - although Dave Gilmour's told me that, and The Beatles have told me that. They said, 'We watched really closely what you were doing in Sunshine Superman.'

"So all that stuff is important, but we also brought all the poetry my dad had read me, all that Gypsy and I had spoken about, and that added up to reflection, introspection and meditation. We had the idea that inside is the answer; outside is the question. Of course, there are many ways to look inside - a bit of hashish, can do that, or you can go in quick with LSD, mescaline, magic mushrooms - but you have to be careful on certain substances, because you don't have a guide. You need a guide. And then you can find the big secret of the whole thing. Which is that there is an invisible world. And everything comes out of that."

That sounds not unlike music, I say. Donovan nods. "Music is the invisible art. The other arts you can see. But music is magic. You can't see it, but if it's made in a certain way, and a human being receives it, it harmonises with the seven parts of the spinal chord, called the chakras" - he gestures to them down my back - "and then people feel at rest, at ease, and in control of their life." He muses on the physical effects of music, and counsels me on super-conscious transcendental vision.

If Donovan's philosophies were progressive, so too were his tunes. His 1966 LP, Sunshine Superman, is widely credited as the first psychedelic pop album. Did he realise he'd created something so significant at the time? "I was the first one to hear that song," he replies, with a typically charming non sequitur. "I picked up my guitar one morning in the flat in Maida Vale." He strums an air guitar and starts singing. "Sunshine came softly a-through my a-window today ..." He begins to annotate the lyrics. "That line was actually a statement because the sunshine was coming through, as I sat there," he says. "'Could have tripped out easy', meaning, I could have done anything - but I'm focused on this one gal that I really need, and it'll take time. It's a love song, but it's also about many other things."

Where did his ideas for the album's far-flung arrangements originate - the exotic baroque flourishes, the sitars? In seeking an answer, I inadvertently upturn my palms, and without breaking eye contact, he takes both my hands. He seems unfazed and I am speechless, so we sit like that for quite some time. "Musically, when I first heard Sunshine Superman, I heard harpsichord," he offers. Then he launches into a verbal trip that veers across Ravi Shankar, producer Mickie Most, the anatomical kinship between humans and saxophones, and touches down in his Glasgow tenement. "My dad played me jazz," he reminisces. "Billie Holiday." He sings Strange Fruit.

Donovan sold millions of records and epitomised hippy-era pop, but he dropped out of music and shrugged off its attendant shackles in the 1970s. If music is magic, the invisible art, then Donovan cast the ultimate spell. He made himself disappear.

There have been rare and welcome revivals since: as the unseen inspiration for Vashti Bunyan's 2008 film, From Here to Before, which documents her 1969 journey from London to Skye in a horse-drawn cart (her goal was to join Donovan in his Hebridean commune, but he'd left by the time she arrived); as the Happy Mondays' spirit guide on Pills 'n' Thrills and Bellyaches (they quote him at length on a wig-out named after him); and now with a new song and career retrospective.

There's something heartening in seeing Donovan embrace invisible wonders, half a century since the Glasgow beatnik tried to Catch the Wind. I'm dazed as I leave him there, waving and smiling, bathed in the sunshine. A super man.

Donovan: Retrospective is released on June 8. He plays Glasgow Pavilion on October 3.

Here are eight movies that feature Donovan's songs.