Two decades since it hit the Earth like a comet (invasion), what more can possibly be left to say about Illmatic? Nas's debut was widely and correctly acclaimed at the time as one of the finest albums ever made, in any genre; and its lustre has only been burnished since. Boasting a concise 39-minute track listing bursting with ideas, the album arrived just as every other rapper seemed convinced the 74-minute upper limit on a CD's duration was a "fill to here" mark that had to be reached, rather than a capacity they didn't need to get near if they had insufficient material ready to include. It has gone way beyond the impact its relatively modest sales might seem to imply: it wasn't certified platinum in the United States until a shade under eight years after its release, yet Illmatic has always been seen as something rather more than a classic rap LP.

The perform-the-whole-album tour (2012), the 10th anniversary "platinum" edition with extra tracks (2004), and this month's 20th anniversary double-disc release, finally uniting the original LP with the remixed singles, are just the tip of an iceberg of the kind of deep and broad acclaim that fills a good swathe of the distance between "sublime" and "ridiculous". In 2010, the Californian rapper Fashawn released a tribute mix tape, Ode To Illmatic. The following year, former Slum Village member Elzhi went a step further with a full-length cover version which he called Elmatic. Also in 2011, the Harlem playwright Shaun Neblett unveiled the first of a planned seven new stage productions that aim to capture the spirit of different hip hop albums. He evidently has been completing them out of sequence, for the first play to be staged was Homage 3. Its subject, inevitably, was Illmatic.

The LP has also been the subject of innumerable pieces of written commentary and critique, the ball set rolling by an infamously controversial five-out-of-five review in The Source (given at a time when the magazine had a stated policy of never awarding the full five mics to anything). It is one of only seven out of the 105 titles currently published or planned in Continuum's 33 1/3 book series about a hip hop album, and was the first in the series about a hip hop album made by a black artist. Perhaps most singularly, it also inspired the 2010 publication of Born To Use Mics: Reading Nas's Illmatic - a book of scholarly essays edited by Michael Eric Dyson and Sohail Daulatzai. The results are far better than a quick glance at the contents might suggest: that studiedly wry tone which academia often adopts as it stoops to embrace popular culture, particularly any form of street art, colours the chapter headings, but works such as Marc Lamont Hill's 'Critical Pedagogy Comes at Halftime: Nas as Black Public Intellectual' and 'All the Words Past the Margins: Adam Mansbach and Kevin Coval Talk Understandable Smooth Shit' are much more in tune with their subject, and much less condescending to their audience, than their titles may appear to imply.

It ain't hard to tell why the album has inspired such devotion and attention; so the purpose here is not to try to sing Illmatic's praises one more time. If a single (re)play of the record doesn't convince, try listening to it with more focus. Every fan can recite the roll-call of heavyweight beatmakers by heart, and even the origins of the samples - an area this series makes no apology for exploring before now, and which will definitely feature in future pieces - are sufficiently widely known to need detain us no further here. What does seem worthy of investigation, though - an area the bulging bibliography is light on - is how Nas managed to create an album that could carry such an excessive combined weight of expectation, attention, investigation and discussion.

In a practical sense, it was straightforward enough, once Nas had met the 3rd Bass man MC Serch and enlisted the then Wild Pitch label A&R and some-time artist manager to shop his demo around New York. True, Def Jam passed, but Faith Newman at Columbia wasn't difficult to convince - legend has it that she refused to let Serch leave her office until a deal had been agreed; she'd been looking for Nas since hearing him on Main Source's 'Live At The Barbecue' but the Queensbridge teen had proved an elusive quarry. The plan was set: get Nas together with the cream of New York's production elite and build a musical vehicle for a rhymesmith who was quickly refashioning his aggressively jocular MO into something deeper, far more rounded, and intellectually nourishing. But, again, the point of this piece isn't to rehash the creation myth: The Source's feature published to coincide with the original release, and which is re-printed in Born To Use Mics, told that tale pretty comprehensively at the time.

There is, though, some interesting elision in the backstory. There's a startling contrast between where Nas got to with the incredibly tight writing on Illmatic's nine full tracks - a bare 21 stanzas shared between them - and where he'd been on those early visits to the recording studio. When 'Half Time' was first released - its title rendered in two words rather than the one it became by the time of Illmatic - on the soundtrack to the Michael Rappaport film Zebrahead in 1992, he was still credited as Nasty Nas; his verses on both '...Barbecue' and on Serch's 'Back To The Grill' reinforce that persona. "When I was 12 I went to Hell for snuffin' Jesus", he declaims on the former, while on the latter he's almost channelling Benny Hill: "This is Nas kid, you know how it runs/I'm wavin' automatic guns at nuns/Stickin' up the preachers in the church - I'm a stone crook/Serial killer who works by the phone book." Yet straight out of the gate on 'N.Y. State Of Mind' the writing on Illmatic is from another dimension entirely: he puts you right there beside him, looking down from that upper-floor window in his mum's apartment in the Queensbridge projects as he watches the users and the clockers, the killers and the police, the life and the death and everything in between. Biggie would talk, later, of going "from ashy to classy", but more impressive was Nas's journey from thug caricature to QB laureate. How did he do that?

There are clues in the parts of the record that are undeniably autobiographical, and in recordings that would follow over the years as Nas kept returning to his origin myth and looking at it all again from the different perspectives that age, experience and his inevitably altered contexts afforded. Whether or not he was "gassed up by a coke-head cutie-pie", as 'Memory Lane' has it, he didn't finish high school. The writing is the work of an autodidact: he learned his techniques from hip hop and the movies. "I did alright at school for a little while," he told me in 1996. "But after I got better at writing rhymes I looked at the rhymes as more promising than a complete education. I concentrated on the rap because that's where I felt more comfortable. I started writing around 1982, but when Run DMC came out in '83, '84, then my mother bought me a typewriter, so I just stayed in the house to write."

Cinema, in particular, was a key driver, yet to be widely accorded its pivotal role in his creative development. Many writers have commented on the visual quality of Illmatic's rhymes, and on the frequent allusions to, and even inclusions of, film in the record's compositions. The opening intro, 'The Genesis', leans heavily on a sample from Wild Style, Charlie Ahearn's fictional narrative set during the early days of hip hop and made in the midst of the culture's late-1970s big-bang birth, which remains one of the most authentic artefacts of the period regardless of the fact that it was never intended to be a documentary. In 'The World Is Yours' - the title itself a Scarface reference (and Al Pacino's iconic creation has a walk-on role in 'N.Y. State of Mind') - Nas is drinking champagne, watching Ghandi. He namechecks The Shining in 'It Ain't Hard to Tell'. In 'One Time 4 Your Mind' he talks about how he'll "watch a flick, illin' an' root for the villain." A one-line joke in 'Halftime' riffs on what were, at the time of its recording, Spike Lee's two most recent films ("You couldn't catch me in the streets without a ton of reefer/That's like Malcolm X catchin' the Jungle Fever"). What's less well documented is how he turned to rap because he felt it was a more realistic means of expressing himself than screenwriting.

"I listened to rap so would write poems, and also I would write scripts - my own type o' movies," he told me during that '96 interview. "So it came to bein' a choice of one or the other, and I went for rap instead of the movies. It was cheaper and easier to rhyme on a beat than to make a movie, plus rap was more me." In a 2004 conversation, he would add: "I always wrote scripts as a kid. I always liked to tell vivid stories."

It's undeniable that a significant part of what makes Illmatic so vital is that screenwriter's eye that Nas brings to his subjects. Just take a look at the third verse of 'One Love' where, after the first two stanzas have been composed - brilliantly, dazzlingly, emotively - as letters from Nas's Queensbridge protagonists to friends in jail (in all of pop music it's difficult to find lines that convey narrative detail with such assured handling of a complexity of emotions in so very few words as: "no time for looking back, it's done/Plus: congratulations! You know you got a son?/I heard he looks like you - why don't your lady write you?"), the final section describes a conversation between the rapper and a 12-year-old child. There are innumerable ways this verse is arresting - and we'll return to it in a few moments - but for now, let's focus on a section towards the end, after Nas has accepted a toke from the kid's joint and is about to take his leave. He breaks his cautionary words to the boy and his admonishment to think on them with two lines of description, included presumably to draw attention to what's on either side by inserting a pause: "I rose, wiping the blunt's ash from my clothes/Then froze, only to blow the herb's smoke through my nose". It's an astonishing detail, a three-second close-up: you're there, right in the heart of this curious conversation, perhaps aware suddenly of an undercurrent of tension and the presence of incipient danger that ought not to attend a friendly chat between a child and a young adult but which has been an unseen visitor all the time regardless. As a piece of writing it's almost too strong - there is a risk that the image takes over the listener so completely that you miss the important stuff that follows - and there is definitely a chance that its writer appreciated how exceptional it was and decided to keep it in even if it might overshadow some of the rest of what it's a part of, because it's just so damn good. But by the time you've heard the song enough times to start framing these kind of internal debates with yourself, 'One Love' has passed from being merely a great rap track on a great rap album to become one of the most important works of art in your cultural life - so either way, Nas has won.

It's not just when he's visual that Nas excels, though. He blends richly drawn imagery with poetic metaphor to add depth and layering to the writing, and those provocative details are an important element of the record's longevity. There's a reason you keep going back to Illmatic that's got nothing to do with familiarity or affirmation or a requirement for a hit of undiluted realness in the beats-and-rhymes department: it's a record that continues to reveal new information even if you're listening to it for the 250th time (and that's going to be a minima for many of us at this point: there've been few of the last 240 months where it's not been played at least once, and many where it's been studied, pored over, dissected and forensically examined a dozen times or more). 'N.Y. State Of Mind' has more than its fair share of examples, whether it's "I inhale deep, like the words are my breath/I never sleep, 'cos sleep is the cousin of death"; or when Nas's character in the song escapes on foot from an ambush by another gang in the project gardens, planning payback even while sprinting for his life: "I ran like a cheetah with thoughts of an assassin". Maybe those lines hit you first time through and later re-playing doesn't reveal any new nuance, but chances are they haunt your subconscious and only start to make a more complete sense after years of maturation and germination down at the back of the listener's mind. Whether they were meant to work in that way or not is irrelevant: the fact that they do means the record continues to compel.

Nas also uses the record as a means of recording an inevitably personal history and attempting to impart some of its teachings. The record is of its time but it transcends its time. Back in that third verse of 'One Love', he's talking about how everything's changed in the brief interval since he was the kid's age, and lamenting the greater efficiency with which death is now dealt on the project corners. But above all that is the sense that there's wisdom he could impart, if only the boy would allow himself to listen. In those lines you intuit that he retains hope - "there's some jewels in the skull that he can sell if he chose/words of wisdom from Nas: try to rise up above". This would not last. A decade after Illmatic another rapper wrestling with the same dilemmas would issue a similar cry of anguish from an environment enveloped in many of the same problems but located a continent and an ocean away: and by then, the belief in the chance of any kind of useful dialogue between the generations was in desperately short supply. "You can't tell them that they're wrong," wrote London-via-Leeds rhymesmith Skinnyman in the track 'Hayden', from his album with the deliberately Illmatic-referencing title Council Estate Of Mind: "'Cos they're all men-child who feel they know what's goin' on/They haven't got no time to enjoy just bein' young or havin' fun/They're out there keepin' it headstrong/They couldn't let Skinnyman come and advise them/They look at me as if my words was patronisin'." Skinnyman's debut, like Nas's, is uncommonly excellent - one of the finest handful of hip hop albums ever to come out of the UK; but its appeal never began to reach the acclaim Illmatic is routinely afforded. Of course, Skinnyman had to overcome the challenge of geography, the lack of a powerful and supportive specialist media, and the tiny size of the UK hip-hop fanbase. And he didn't have the resources of a major label to help him get across. Nevertheless, Council Estate Of Mind comes from a bleaker time: its excellence and its power are unlikely to make as wide a set of connections because its frank appraisals of the era it was made in leave us unconvinced that the future will be better. Illmatic, even in the moments it might today feel like a period piece or a curio from an age of lost innocence, never quite lets us give up on those hopes.

There's another important way the writing on Illmatic has enhanced its reputation and prolonged its impact and influence. Again, whether he fully understood at the time is moot - and, in many ways, is unimportant, because what's happened since has happened regardless of whether Nas imagined it might or not - but in Illmatic he managed to create a kind of a renewable energy source: a hip-hop equivalent of solar, wind or tidal power. He's been able to keep going back to the album and using it as a thematic, conceptual and inspirational fuel, connecting whatever he's doing at whichever point in his career back to this touchstone recording and the forces that motivated him at the beginning of his artistic journey. He's done this on many occasions, often in pretty obvious ways, but, beyond noticing it, he and his audience haven't really spent much time going into what it means or why it might happen so often.

Some examples, then. Most unignorable is the titling of his 2001 album, Stillmatic, a move almost guaranteed to heap pressure on the rapper's shoulders. At the moment that record was released, Nas's career was at a pivotal, perhaps even critical, point: he was in the middle of the war of words with Jay-Z that became one of hip hop's most celebrated on-record feuds, and was returning after a relatively lean patch in which his most recent albums (I Am... and Nastradamus) had left rather more fans and critics disappointed than impressed. Evoking his classic debut seemed a risky strategy. Yet in his moment, arguably, of greatest need, it was the spirit and the legacy of Illmatic that he sought out and chose to immerse himself in.

Even if that particular nod was an aberration in its obviousness and in its specific context, you only have to scratch the surface of the rest of the man's career to see how much he's come to rely on Illmatic to, as it were, put a battery in his back, a difference to Energizer. True, the title of the album Street's Disciple is technically a pre-Illmatic reference - those are the first two words in the first line of his first appearance on record so the title is designed to convey the sense of a return to first principles: but his 'Live At The Barbecue' verse plays in the background on 'The Genesis' so those are still the first four syllables of lyrics that we hear on Illmatic. Street's Disciple also contains the song 'Thief's Theme', the title of which comes from a line in 'The World Is Yours' (a song in which he takes time out to muse on what he might name a child not yet born when the album was released. He says he's trying to find "a word best describin' my life to name my daughter". She's called Destiny, which is a concept so profound on so many different levels that we probably need another piece of similar length to this one to fully unpick them all). On Hip-hop Is Dead there's a track called 'Carry On Tradition', which is a reference to a line in the one Illmatic verse Nas doesn't perform (AZ's opening stanza on 'Life's A Bitch'). He included 'N.Y. State Of Mind Part II' on I Am..., and the Nastradamus track 'Project Windows' links back to the vignette he gives us in 'Memory Lane', of the poet as a young man watching from that same window that "faces shootouts, drug overdoses". Then there's the lyrical references that are a little more subtle, such as "Please God, let him spit that 'Uzi in the army lining',"or "Needed time alone to zone/The mack left his iPhone and his 9 at home" - both found in the opening track to his untitled 2008 LP.

Then there's the phrases, images and metaphors sprinkled throughout Illmatic that have passed into the wider hip hop vernacular - little bon mots that act as a kind of insider's code, a flash of a smile from one initiate to another, a signifier of authenticity: occluded messages passed between the anointed, like the one in the first sentence of this piece, and the one at the end of the first sentence of the last paragraph above. That Dyson/Daulatzai book title is another fine example -four words from 'N.Y. State Of Mind' that, removed from the song and written in 72-point type, start to morph from being a simple poetic image and acquire the quality of the sort of statement that you more usually see translated into Latin, chiselled in granite and parked above the door of a public building. Illmatic is littered with groupings of words that have gone on to become titles or memetic concepts, referenced by other rappers and by other writers in different disciplines. "Beyond the walls of intelligence life is defined." "Born alone, die alone." "When it's my time to go, I wait for God with the fo' fo'." "Somehow the rap game reminds me of the crack game." "Half man, half amazin'." The effect on the wider hip-hop conversation is not unlike the way phrases from Shakespeare routinely pepper everyday contemporary English.

A degree of mysticism and the notion of Nas acting as a prophet also feed in to Illmatic and help explain some of its magnetic or hypnotic effect. 'It Ain't Hard To Tell', its three verses of blunt-smoke-infused hallucinations, is one to return to and keep pondering ("I drink Moets with Medusa, give her shotguns in hell/From the spliff that I lift and inhale"), but even Nas himself has kept going back to the album as if he doesn't fully understand everything he said or implied on it and needs to keep trying to work it all out. "I decipher prophecies through a mic and say 'Peace'," he rhymed in 'Memory Lane' - but he's never really been able to give up on reinterpreting some of the coded messages he left for himself in his own lines. That Scarface reference in 'N.Y. State Of Mind', for instance, kick-started a chain reaction in which we get to see and hear the supernatural at work in the poet-rapper's psyche.

Almost a decade on from Illmatic, Alicia Keys managed to corral Nas and Rakim to create a new version of the song which she called, in a comingling of Queens hip-hop references, 'Streets Of New York' (the titular nod is to a key formative influence on Nas - Kool G Rap - whose 'Streets Of New York' is a clear ancestor of some of Illmatic's social realism; for all that he is most often compared to Rakim, G Rap is another important reference point, and even as recently as Streets Disciple he homages the Juice Crew all-star by adopting his chewed-word flow and rapping over the same Billy Joel riff DJ Polo and Marley Marl hooked up for 'Road To The Riches'). In his verse on the Keys remake, Nas consciously referenced his original, but updated the metaphors for the post-crack, Iraq-war era. So "At pain, I'm like Scarface sniffin' cocaine/Holdin' an M-16: see, with the pen I'm extreme" became "At pain I'm like Saddam Hussein/Still alive - lookin' at his dead children's burned remains."

In a subsequent interview, Keys told me that those lines were recorded at least a week before Uday and Qusay Hussein died in a shootout that ended with the building they'd been hiding in burning to the ground, and images of their partially incinerated bodies were broadcast around the globe. "I don't know how that happened," she said. "It was the craziest thing. We were in the studio, he came in and dropped a verse, and I was like, 'Wow, that's a crazy thought! It's ill that you're thinking like that!' It wasn't until a week or two later, I'm in the gym, running on a treadmill and the TV's on: they're talking about Saddam Hussein and his kids, and all of this stuff that had happened. And the first thing I'm thinking was this lyric that he said. This was at least a week before. It wasn't 'til a little later that we got a chance to speak on the phone, see how he was doin', and I was like, 'What was that about? Why did you say it like that? How did you know?' And he was like, 'I was just speculating about how that would feel, for him, and I was trying to describe that angst in the rhyme'."

Not that all of Illmatic is prescient or erudite or sage and quotable - contrary to what some of its legend might push you towards believing, the record is by no means perfect (and of course it's debatable whether perfection can exist in art, or whether it would be a desirable quality even if it were possible to achieve, but those are discussions for another time). It's worth noting that nobody has, as yet, chosen to title a book of learned rumination after the very next 'N.Y. State Of Mind' line that Dyson and Daulatzai chose (the full couplet is: "It's only right that I was born to use mics/And the stuff that I write is even tougher than dykes"). 'Halftime' offers what is some way short of a sophisticated understanding of the nature of human sexuality ("Versatile - my style switches like a faggot/But not bisexual, I'm an intellectual"). But these tracks do fairly bristle with weighty, sometimes enigmatic but often very deep ideas expressed, usually, with uncanny precision, which have gone on to become a kind of hip hop equivalent of sitcom catchphrases. The moment you hear them they identify the artist and the album, and its historic moment - and that latent power is one Nas has never turned down an opportunity to draw from.

Conventional wisdom - perpetrated over the years, it's only right to admit, by the likes of, um, this writer - has sometimes held that Nas has been in thrall to Illmatic; that the record is as much curse as gift, an albatross he unknowingly slung around his neck at the beginning of what has turned into a remarkably, possibly unprecedentedly durable and consistent career. By making such a totemic record right at the start, he has been doomed to live in its vast shadow, always bound to have every new record compared to his debut and usually held to be inferior. It's easy to see why that view gained ground, particularly given the general dissatisfaction many fans and most critics felt with the three records that followed it (and in particular the way parts of the 1996 second album It Was Written moved away from observational tales of Queensbridge life to discuss imagined Mafia storylines), none of which seemed to share much in the way of thematic or conceptual cohesion with their forebear. To be fair, we should have seen it coming: he'd flagged it up clearly enough when, in 'Memory Lane', he showed how hard it was to separate the violence he watched on a TV or cinema screen from what he saw on the other side of his project window ("check the prognosis/Is it real or showbiz?/My window faces shootouts, drug overdoses/Live amongst no roses - only the drama"). But that doesn't explain why Nas should have so often and so obviously sought to remind us of Illmatic, to draw attention back to it, to all but make it impossible to listen to his latest work without thinking of his masterpiece. Rather than try to establish a distance between his latest music and his first LP, Nas has continued to demand that we always remember where he came from and the heights he scaled at such an early age.

In a December, 2001, interview, I asked him whether he felt he was putting himself under pressure by calling his then-new album Stillmatic, and his answer, typically, was to turn that question around. "No," he said, "I feel I've put a lot of pressure on my fans to see who's gonna stick by me. It's really for the minds of the fans. To me, it was making an album the way I want to do it, and calling it Stillmatic was just to remember who I am and what I represent and what I'm doing. For the fans it's a number of things."

Speaking on the eve of the release of Street's Disciple in 2004, he got into this theme a little more deeply, arguing that the record fans thought they wanted in 1995 - a follow-up to Illmatic that sounded like Illmatic and continued the themes of Illmatic with writing similar in style to that he had developed for Illmatic: a record that he actually got quite a bit of the way down the road towards making but which was never completed or released - would have left his career stillborn. I had asked him about the expectations people had had for the second album and the two records he released in 1999 (I Am... and Nastradamus), in particular the backlash that seemed to develop against his move towards more obviously imagined narratives in his songs. He wasn't having any truck with the notion that he should be giving his audience more help - the question of whether what we hear on the record is really him, or whether it's a character he's portraying in a piece of musical drama, is one he doesn't feel it's his job to answer ("I think that's up to the listener, that's their issue," he said. "That's their problem to figure out"). But asked about the change that came over his writing post-Illmatic, he was lucid, forthright and, I felt at the time and still feel on re-reading these words a decade on, honest.

"Well, you never know when people are gonna give up on you," he said. "You never know when you're gonna lose 'em. You never know 'til the record is released, you just have to go with your gut feelings. It's important to balance your name in the commercial world without compromising what you do. You can't listen to what people are saying because they don't know, really: they're not in your mind when you're doing that. They're not sharing your mission. They're not there creating. They're not gonna keep your family fed, they're not there to motivate you. So you can't listen to what they say too much - you've really got to stay on your path.

"I recorded a few songs for my next album after Illmatic and they were grimy, they were hard," he continued. "Something happened - the songs leaked out with just one verse on 'em. Marley had them on the radio, and it just changed my flow. So what happened was, I said, 'Nah, this has to be a masterpiece. And I'm saying more than I'm walking down the road smokin' weed with my pants saggin' and my Timbs, in my left is my gun, got a bitch on the side'. I said, 'Nah, that's not who Nas is. They think they know who Nas is, but I can't wait to show 'em who Nas is'. And I got together with the right team to put together an album that represented who Nas is - 'If I Ruled The World', 'Black Girl Lost', the things who I really was. And thank God I did that. Because I realised the kids were saying 'Give us another Illmatic', but years later I said, 'Damn! If that was the next album, I would have been finished'. Not just because it wouldn't have sold, but more importantly because I wouldn't have had any drive to continue. Because I would have felt like, 'Damn, my body of work only says I'm about gettin' high and shootin' dice and hangin' on the block - and that's it'. And because that's the way I went to the studio to record it, just talking about everything without a care in the world for song structure, musical arrangement, clear and crisp sounds, taking your imagination to the next level - it was just one side. So I thank God I didn't do that, man. It would have been cool if I had been able to do that album, then still been able to make It Was Written next. But I just think that I would have lost the drive to do it. If I would have just put out two albums with basically the same grungy sound, it would have been, 'If this is who Nas is, then I'm finished, I've had enough, I'm not inspired'."

So although they weren't the records Illmatic's devotees thought they wanted to hear, the next few Nas LPs were the ones that the artist had to make, without which the later - and generally pretty well-liked - records couldn't exist. So without them his place in hip hop history might well have been as a one-album wonder. Stillmatic got him his second five-mic review in The Source, though quite how that appraisal was arrived at remains something of a mystery - few fans would seriously argue that Stillmatic is one of his two best albums. But the run he's been on since has been unimpeachable.

God's Son found him getting back on to some of those old-style breakbeat tracks and rapping about subjects close to his heart - whether playing with rhyme over the Incredible Bongo Band on 'Made You Look' or getting deep into the occult behind-the-scenes history between him, Biggie and Wu-Tang on 'Last Real N*gga Alive', the record fizzes with a renewed sense of purpose. The 90-minute Street's Disciple has its flaws, but it also contains some of Nas's finest ever moments - American Way is as righteously, furiously angry as anything in the Public Enemy catalogue, its four minutes giving the lie to any suggestion Nas was 'hood-centric, introverted, apolitical; 'Bridging The Gap', one of several collaborations with his jazz trumpteter father Olu Dara, brilliantly sketched both men and their musics in to the lineage of the blues.

He followed it with two deeply conceptual and often totally misinterpreted albums - the lament for a culture being sold down the river that he provocatively titled Hip Hop Is Dead (and which listeners round the world queued up to misunderstand, blasting Nas for saying things that he hadn't said, probably hadn't even thought), and the 2008 meditation on racial politics he released without a title, after his label told him US chain retailers weren't going to stock it if he called it N*gger as he had intended. Both are excellent and speak loudly to their maker's flair for innovation, whether he's adopting a Humphrey Bogart persona and voice to investigate the murder of the genre he loves or rapping over unaccompanied French film soundtrack piano. A fine and ambitious collaboration with Damien Marley and the post-divorce braggadocious introspection of Life Is Good round out what is, by any rational standards, one of the most consistently high-quality, provocative, creatively questing and artistically successful of hip-hop catalogues.

And even if you can't point to any one of those records and say, definitively and empirically, that it's better than his debut, there's a decent case to be made that Nas may well be the greatest rapper of all time. If you combine longevity, consistency, baseline quality, technical and thematic impact and innovation, global cultural importance and plain old value-for-money (an album every two years, and notwithstanding the less-than-glowing consensus on numbers three and four, no duds), who else is there?

Chuck D likes to call PE the Rolling Stones of rap, but Nas is the music's Bob Dylan: forever chained to a pop-culture-altering past, but never willing to submit to his audience's preconceptions and deliver the expected next record - always trying to do something new, to swim against the tide, to keep on moving forward because he knows that when he stops, he dies. He has the audacity to believe that the rest of us need him to do this - he said as much on 'Memory Lane' ("I reinforce the frail with lyrics that's real") - and our continued fascination with and love for this remarkable record seems to bear him out. Illmatic is more than just the magnificent cornerstone Nas built a legacy around - it's the deep wellspring from which it all flows.