Almost 30 years ago, in a classroom in the Jung Institute of Evanston, Illinois, Dr Murray Stein delivered a series of lectures that have had an unexpected impact on pop history.
The topic was Carl Jung, the founding father of analytic psychology, who proposed and developed the concepts of extravert and introvert personalities, and the power of the unconscious.
Dr Stein had condensed Jung's theories, originally scattered across 18 volumes of his collected works, into an accessible primer for students and trainee psychoanalysts.
His course became a book - Jung's Map Of The Soul - which is now recognised as one of the best introductions to the concepts of analytical psychology, reprinted 15 times in English, and translated into dozens of languages.
Much to Dr Stein's surprise, it is also the basis of the latest album by South Korean pop phenomena BTS - set to go to number one on both sides of the Atlantic this week.
"A couple of months ago, a Japanese student told me he'd discovered that the BTS website was recommending my book," he tells the BBC from International School of Analytical Psychology in Zurich.
"I said, 'What is BTS?' and he told me, so I looked them up online, read a little bit about them and left it at that.
"Then about a month ago, the same student told me they were coming out with a new album titled Map Of The Soul: Persona.
"I was floored."
It's not just the album title, though: BTS's lyrics delve into Jungian concepts of the psyche, ego and collective unconscious - with a particular focus on the idea of Persona.
"Persona is a reference to the theatre," explains Dr Stein. "It's the Latin word for the masks that actors wore on the stage - and we all put on masks, in a sense, when we go out into public.
"It's part of being a social animal: Our need to get along with other people, our need to be polite, our need to be part of a group.
"In some cultures this is more important than others, and I must say in the Asian cultures of Korea and Japan, where BTS originate, persona is an extremely important part of their lives.
"How you present yourself, how you address other people, how you locate yourself in the social world - as a younger brother, or a student, or a professor - all of this is really very prominent in their consciousness and their functioning as people in their society."
BTS plunge straight into this concept on Map of the Soul's opening track.
"'Who am I?' is the question I've had all my life / And I'll probably never find the answer," raps Kim Nam-joon, discussing how praise for his on-stage persona stops him addressing his flaws and getting to know his true self.
In the video, Kim confronts a giant version of himself, illustrating how his persona has overshadowed his ego; and performs in a room filled with mirrors, each reflecting a suppressed aspect of his personality.
Dr Stein recognises the band's struggle to balance their public and private life as "the persona trap" - a condition that can trigger serious psychological problems.
"It's a very important topic because young people who find their personas inadequate, or feel like they're not fitting in, are very vulnerable to mobbing (bullying) or to suicidal acts - so I think BTS addressing that is very timely and important for their audience."
The band continue to grapple with these ideas of identity throughout Map of the Soul: Persona.
Mikrokosmos talks about deriving self-worth from within; while Jamais Vu looks at our tendency to repeat the same mistakes over and over again.
Dr Stein explores the lyrics in-depth in a recent episode of the Speaking Of Jung podcast, where he explains how Map of the Soul is an album full of "longing and struggling for authenticity" that resolves on the final track, Dionysus, with the band "breaking out of persona traps" and reaching an awakening.
It's an unusual area for a pop record to explore - but K-pop artists often adopt "very unique and specific" perspectives, says Billboard columnist Jeff Benjamin.
"I've seen K-pop concepts that have ranged from Greek mythology to exotic animals to religious characters to Nick Cannon movies, but I don't think I can recall a popular K-pop act discussing themes of psychology and philosophy in their lyrics.
"And if they did, it wasn't a band at a level anywhere near BTS or they weren't as explicit as the group is on Map of the Soul."
"BTS are quite unusual," agrees Dr Haekyung Um, a senior lecturer and specialist in Asian music at the University of Liverpool, "but if they weren't a K-pop band, I don't think this would be such a big issue.
"The way that K-pop is perceived, especially in the West, is to put it in a little box where everything is manufactured and artificial and the artists do not have their own voice.
"But if someone like Joni Mitchell had lyrics about the map of the soul, I don't think it would make a really big headline."
Dr Um, who grew up in South Korea, adds that self-examination has long been a feature of Korean folk music, through artists like Chung Taechun and Kim Kwangseok, and that philosophy is a mainstream concern in the country.
"Knowledge is a very important asset, and Koreans enjoy reading quite heavy philosophical texts," she says.
"Knowing and understanding something profound is seen to be really important. It will increase your self-confidence and value, and you become a more enlightened person.
"So when you go to the best-sellers display in Waterstones here in the UK, you wouldn't find books on philosophy - but you would in Korea."
She also highlights the benign influence of BTS's record label boss Bang Si-hyuk, who studied philosophy before becoming a songwriter, on the band's output.
"I don't think he's the one who told BTS what to do, but he's very talented and very intelligent and this is a very clever business tactic, to sell all these books together with the music; and if it helps the audience to appreciate and enjoy the literary side, why not?"
For Dr Stein, the release of Map of the Soul has given him a crash-course in K-pop and modern fandom. For the last three weeks, he's been busy replying to questions from the BTS Army about the band's lyrics and his own books (which have soared up the charts).
"It's kind of taken over my life," he laughs. "I'm 75 years old. I don't know very much about pop music, except it sounds very noisy."
Nonetheless, he's emerged impressed with BTS's understanding of psychological health.
"I've been told death and aggression are big themes in music - you know, rappers doing their thing in a very assertive way - so this seems a very wholesome and positive direction for a pop band of their magnitude.
"They have millions of followers and for them to be putting out this kind of message is very encouraging, in this world where we're constantly struggling."
And Jeff Benjamin says BTS's exploration of Jungian psychology isn't likely to end with this record.
"BTS have a history of releasing albums in trilogies, and they said Map of the Soul: Persona would kick-start a new chapter in their career, so I definitely feel like we're going to see at least one more Map of the Soul album.
"If 'Persona' was the first aspect of Jungian theory they explored, maybe we'll get Map of the Soul: Shadow, which I'd expect would explore the darker sides of the BTS members' psyches.
"Actually, that would be a truly fascinating side to explore and something else we have not seen from the K-pop scene either."
Dr Stein agrees: "I think this album is laying the groundwork for further developments.
"We'll have to see what they come up with next."