What is art? It’s a question whose answer has evaded us for centuries. Attempts to define it clearly have never worked because, like the wind, we can see and feel it all around us, but can never quite pin it down.

Frustrating as this might be, mostly we know art when we see it; this elusive quality is what makes it so fascinating and versatile. In turn, that means that art is forever reinventing itself and can appear in the most surprising forms, especially these days when fairly mundane items created for a different purpose are remodelled or redefined as art – folk art, for instance.

A good example of this is the EU entry gate sold at auction last year when the entire contents of Terminal One at Heathrow came up for sale. The hammer came down at £1000 and no one thought any more about it until the Royal Academy Summer Show opened a couple of weeks ago and there it was, transformed by Banksy into a new work of art combining humour and political statement. The gate itself looked almost the same as it had done when auctioned off. Banksy had added a shutter and painted the words Keep Out on it, rendered as Keep Ou because his signature rat had removed the T to use it as a hammer to break the padlock at the bottom. Not to everyone’s tastes, maybe, but with Banksy’s transformative touch, this work of art will be worth a great deal more now than its original auction price.

In many ways, what must be one of the most astonishing auctions of recent years has just taken place. The lot in question? The gun with which Vincent Van Gogh is supposed to have killed himself.

The astonishing thing about it is that even the auctioneer selling it was clear that no evidence exists to show that this was the actual gun, yet it made the equivalent of £144,000.

The troubled artist died a few days after walking into a field near Auvers-sur-Oise and shooting himself in the chest. It was July 27, 1890. No one knows what happened to the gun, but in 1965 a corroded revolver was dig up in a field near the chateau where he had been staying and tests indicated that it had been in the ground for between 50 and 80 years – so about right.

That’s as far as it goes, so the buyer was acting in faith, just as pilgrims making their way to shrines that house anything from St Catherine’s finger to a thorn from Christ’s Crown of Thorns are also taking it on trust that these relics are genuine.

Ordinary everyday items acquire a unique symbolic status when they have been touched by inspirational figures like Van Gogh, who will remain forever out of reach themselves. Akin to holy relics, they hold a special attraction for bidders at auction.

It’s that time of year when sporting heroes come to the fore again. Lewis Hamilton continues his dominance of Formula One, Rafa Nadal wins his 12th French Open title – aged just 33!! – and England’s Women’s Football team leads its groups in the initial stages of the World Cup… I could go on.

Along with music, sport serves a vital role of bringing all sorts of disparate people together in a common cause – something we need now more than ever – and it is not always the obvious candidates who become heroes. Look at how the Mexican underdog Andy Ruiz Junior, trolled for appearing overweight and flabby, took out Anthony Joshua in the boxing ring at the beginning of the month.

Sport is not just about achievement and titles; above all, it is about character. That’s why Babe Ruth’s baseball jersey sold for $5.6 million at auction on June 15, to become the most expensive piece of sports memorabilia in history, breaking the previous record, which was also for a Ruth jersey.

Known to have survived largely on hot dogs and beer, Ruth was a big man but, along with his penchant for cigars, he could not have been in the best shape and, indeed, he died of cancer aged 53. This didn’t stop him from recording 714 home runs across 22 seasons, winning seven World Series.

The greatest slugger of them all remains a hero, transcending his sport to enter America’s national conscience as a cultural icon.

One of the most important factors affecting value for many items going to auction is the maker. This is particularly true when it comes to fine art, whether drawings, prints, paintings or sculpture.

It can be quite hard to get your head around how this works with conceptual art – an idea conceived by an artist but actually executed by their assistants, without them having a hand in its manufacture. Certainly, with Old Master portraits, many of which were completed by studio assistants, it is only when we know that the most important parts – usually the face and hands – were painted by the master himself that we can call it an autograph work and the price shoots up. This is why catalogue descriptions can range from ‘attributed to’ all the way down to ‘Circle of’, ‘School of’ or ‘Follower of’.

The magical hand of the artist has made all the difference to the star of this year’s Royal Academy Summer Show Keep Ou. Another comment on Brexit by the mysterious Banksy, he has used as his ‘canvas’ an arrivals gate from Terminal One, Heathrow, sold off last year as surplus to requirements. I have no idea what the price was when the hammer fell then, but you can bet that the RA price tag is considerably higher.