LET me start by saying that, although this tale is an old and well-kent one, it very likely belongs in the realm of folklore rather than history.

Indeed, like many folk tales, there is more than one version.

But that said, it contains its own truth, as all folk tales do, and could be true.

It begins with a fishwife’s journey to Edinburgh. I like to think she hailed from a fishing community in East Lothian, for she is not named or given a place of residence in the tale.

This is because she’s not the main character in the story. The main character was David Hume, the 18th-century philosopher and historian.

He was a controversial figure at the time because of his religious scepticism, which limited his academic career. But enough has been written about him and his works.

The tale came to my mind as I was venturing home after seeing traditional storyteller James Macdonald perform eloquently during one of his Fringe storytelling performances. He moved in and out of Gaelic, prose and song, as he took us on a journey through Scotland and the stories and tunes of his younger years.

Afterwards, I headed to Princes Street to catch the 26 back home, my head still running with the folktales I’d heard. I decided to soak up some festival street atmosphere so went via the Royal Mile but quickly regretted it. As soon as I turned into the Lawnmarket, I encountered a heaving mass of humanity. It was, for me, too busy to enjoy.

So, to avoid the crowds, I cut down a close to James Court, then onto The Mound, where I could descend to Princes Street and get my bus. This was when I remembered the tale, and it now ran in my imagination as I walked in the footsteps of David Hume - well, not exactly in his footsteps, because The Mound didn’t exist as is does now in his day. But nonetheless, he did follow this route.

David Hume used to live on James Court, moving there in 1762. It was the fashionable part of town, where the wealthier tended to live.

But in the 1760s, Edinburgh was bursting at the seams, and the overcrowding and lack of space meant that, even for the better-off, accommodation was cramped. Insanitary conditions added to the experience and thus the New Town was begun, a dream of new classical and ordered houses, with space and fresh air, in stark contrast to the filthy, overcrowded ancient wynds and closes of what was soon to be called the Old Town.

Hume staked his claim to be part of that dream by buying a plot on the edge of St Andrew Square and commissioning the construction of a grand house.

As the building of his new home was progressing, he liked to regularly check on it, but there was a problem: an ancient loch called the Nor Loch used to separate the Old Town from the ridge on which the New Town was being built. It had recently been drained, but in Hume’s time it had become a rancid bog, not yet the beautiful Princes Street gardens we know today.

The Mound didn’t exist either, but there was a precarious pathway across the bog along the same route. It was along this shortcut that David Hume was one day walking to inspect his nearly completed New Town home.

And he slipped into the mud!

Try as he might, he couldn’t get out. He was a man of significant proportions by this time and he looked like an upturned turtle writhing in the bog. He needed help, so he called out to passersby, but when they realised who he was, they declined, saying this was his punishment for not believing in God!

This is the moment the old fishwife comes into the story. She was a godfearing and deeply religious woman, and seeing a man in distress, immediately went to his aid, accompanied by a younger friend.

But when she recognised him, her helping hand was withdrawn and she turned to her friend.

“He’s that unbeliever, leave him… the Deil has him, let the Deil keep him!”

They were about to walk away when Hume appealed to the old fishwife.

“Madam, I see ye are a guid Christian, does not yer creed teach ye tae love yer neighbour, and tae be a guid Samaritan?”

The fishwife turned and walked back, surprised such an ungodly man used such Biblical references.

“Aye, weel that it does, but the guid lord has seen fit tae confine ye tae this bog fir yer sins, and I cannae pu’ ye oot, unless ye tae become a guid Christian, and prove it by reciting tae me the Lord’s Prayer and the belief.”

Bystanders waited with bated breath to hear Hume rail against religious superstitions and refuse on principle to do the recitations. But he was in a very sticky situation, literally, and his sense of humour and philosophical reasoning overcome his principles or pride.

So, to the surprise of the old fishwife, he recited the texts perfectly from heart, and with such conviction that she relented and helped pull him out!

“Thank ye madam, ye are indeed a guid Christian,” he may have said, and he sauntered on his way covered head to toe in stinking mud. Probably the old fishwife knew she hadn’t really converted him, but nonetheless she had kept her word.

It is said that Hume told this tale himself when he had moved into his new home in what became known as South St David Street. There is a tradition the street was ironically named after him, and maybe it was.

I passed the the street as I headed to the bus stop. Hume died here in 1776, aged 65 and still an atheist, although he wouldn’t recognise the street now. I glanced up at the nearby Mound, which he wouldn’t recognise either as it was created after his death. But I’d followed his old route.

Over one-and-a-half million tons of earth, which had been dug up while making the foundations of the New Town houses, was dumped to make the crossing over the boggy remains of the Nor Loch which Hume had fallen into. People just called it The Mound, and the name stuck.

I couldn’t help thinking that Hume would enjoy walking up The Mound to the Old Town to view the statue of himself. But I think, also, there should be a statue of the fishwife, perhaps halfway up the mound, close to Assembly Hall, which is the meeting place of the General Assembly of the Church of Scotland.

She was, after all, according to Hume, “the most acute theologian" he had ever encountered.