Who was the best driver in Formula One history?
The question is a staple of forums, comments sections and social media. Was it the star of the 1950s, Juan Manuel Fangio, who won 24 races of 52 entered and won five world titles?
Or perhaps it was triple champion Jack Brabham, the only man to ever win the title driving a car bearing his own name. Jim Clark won a then-record 25 races before he was killed in a crash aged just 32 in 1968 and is surely another man worthy of consideration.
The late 1960s and early 1970s saw Jackie Stewart emerge as the top driver of the day. He and Niki Lauda each won three world titles and are two more names to add to the list.
Alain Prost won 51 races and four championships during the 1980s and 1990s and after he retired seven-time champion Michael Schumacher dominated the sport for more than a decade.
But the driver most often mentioned when people discuss the best of all time is Ayrton Senna, who was killed at the age of 34 during the 1994 San Marino Grand Prix.
His is still the most powerful and evocative name in F1 even today, as we approach the 20th anniversary of that dark day at Imola.
Fans still talk about his greatest drives, DVDs and documentaries continue to be released and merchandise bearing his name and image continues to sell all around the world.
Senna frequently tops polls asking who the greatest of all time was and many a list by many a fan, driver or analyst places him at No. 1.
But was he really the greatest?
Some consider statistics to be a good measure of greatness, so they're a good place to start.
Here's a comparison of the eight men mentioned above in seven key statistical measures of greatness: entries, wins, win percentage, poles, pole percentage, grand slams and world championships.
The top three in each are highlighted gold, silver and bronze.
What immediately leaps out is Michael Schumacher's dominance when it comes to pure facts and figures. He's in a different league to everyone else with wins, has the most poles and championships and started far more races.
In the world of percentages, it's Fangio who stands head and shoulders above the rest. His nearest rival, Jim Clark, is at least 10 percentage points lower in the two categories listed.
But Clark does have a remarkable number of grand slams from only 73 races.
Senna has an excellent record but against these men it doesn't stand out. Depending on how you weight each category, Senna comes out somewhere between third and fifth.
Statistically, Senna is not the greatest of all time.
But statistics tell only the most shallow of tales. They don't tell us how a driver won, only that they did.
They make no mention of who had the best car and when. We can't tell from the numbers which drivers had the quickest teammates. The table above says nothing of how many race wins were lost to reliability issues.
Statistics don't reveal that Clark and Senna's careers were cut short by fatal crashes, that Jackie Stewart voluntarily retired in his prime or that Fangio was 39 when he made his F1 championship debut.
They don't give credit to Brabham for forming his own team (with a partner) and turning it into one of the success stories of the 1960s.
And most of all, we can't look at statistics and pick out the moments of brilliance that are the difference between a good driver and a great driver.
Or in this case, a great driver and the greatest driver.
An alternative method of assessing the merits of each driver is to actually watch the races, take in and analyse all the information and come to a conclusion based on what was seen—not what was written down afterwards.
This method should always produce a more accurate and real reflection than statistics, but it too has flaws.
The biggest is that "greatness" measured in this way is subjective. What does greatness mean to you?
Is the greatest driver the man capable of hitting the highest peaks, or the one who consistently performs at the highest level? The one with the most natural talent, or the one with a little less but who made more of it?
If each wins 30 races and three championships, who is "greater"—Driver A or Driver B?
Another problem with this method is time. Extensive coverage of historic races just isn't available, so the only people who could give a truly accurate assessment are those born before 1940 and who have religiously followed F1 all their lives.
At the turn of the millennium, two such men were asked by The Independent who they thought was the best of all time. Stirling Moss had no difficulties, replying, "That is surely the easiest question I've ever had to answer. Juan Manuel Fangio."
Ken Tyrrell, then 75 years of age and a team owner for more than 30 years, had a different opinion. He said:
I find that's an impossible question. To say that Fangio is better than Ayrton Senna is ridiculous. I couldn't say that any one of them was better than the other unless they were driving the same car at the same time, which is impossible.
This comment brings us nicely to the final problem. As Tyrrell said, we can't judge the relative quality of two drivers unless they are driving the same cars and facing the same opponents.
Fangio raced front-engined, wingless cars in a t-shirt and leather cap against the likes of Stirling Moss and Alberto Ascari. Senna drove precision-engineered racing machines against men of the calibre of Alain Prost and Nigel Mansell.
Comparing the two, or any of the other drivers commonly called the greatest, is extremely difficult unless they raced together. To some, it's impossible.
But that doesn't stop us trying. Polls on the subject are a constant presence on message boards and most F1 publications and websites have, at some stage, produced their own rankings.
BBC Sport's list was created by asking their F1 team what they thought. Senna topped the rankings, with Fangio second and Jim Clark third.
The Daily Mail sports team ranked Schumacher at No. 1, followed by Senna and Fangio.
In a knockout tournament-style series of polls on F1 Fanatic resulted in a final between Senna (who scored 57 percent) and Schumacher (who scored 43 percent).
And perhaps the list to which most attention should be paid is one produced by Autosport. In 2009, the magazine asked 217 then-current and former F1 drivers who they thought was the greatest of all time.
The winner, as it so often is in such polls, was Ayrton Senna.
But even that list is only so much use, because an overwhelming majority of those who voted either weren't born or were too young to really remember the 1950s and 1960s. Some won't even have seen action from the 1970s or 1980s.
And none were gifted with the ability to accurately compare drivers of different eras.
So back to where we started—where Senna stands among the all-time greats.
He was certainly the quickest driver of his era. Prost was an exceptional driver and had very good race-craft but in terms of raw pace even he couldn't touch Senna.
Beyond that, we're into uncertain territory. We cannot judge whether Senna was greater than Clark or Fangio because we never saw them race one another.
So was Senna the the greatest of all time?
But then, neither was anyone else.