Five years ago, a Chicago-born student came to the United Kingdom to work as an intern at the British parliament.
It was to be a life-changing experience.
Mullins-Silverstein, who was studying international relations and economics at Florida State, had never been to Scotland.
As far as he knew, he had no Scottish ancestors or relatives. And he had little idea about its history, how it had become part of the U.K. some 300 years before and the reasons behind the modern-day campaign for independence.
"I came in fresh, I didn't really understand the issues at all," he told msnbc.com, recalling how he had asked his new colleagues "why does the SNP exist?" with the naive frankness of a newbie.
But within weeks, he found himself fired with a passion for the cause that was to bring this 29-year-old African-American back from the U.S. to work full-time for the party in 2009.
"It took me two or three weeks to really get on board. I started looking at the (political) arrangements and how the U.K. was working," he said of his four-month internship.
What Mullins-Silverstein found was "entrenched inequality."
"The U.K. is not a nation of equals," he said.
Tax rates, for example, were set at levels that worked for London and the south of England, but not necessarily for the more rural Scotland, Mullins-Silverstein said. "In Scotland, they cannot even set their own speed limits," he said.
'Got me emotionally'
He was riled by what he saw as an over-centralization of power in the government offices in London and further incensed by the arrogant dismissal by some critics of the idea that Scotland could survive without the rest of the U.K.
"When I first came over here, the talk was that if Scotland went independent it would go back socially and culturally — that didn't make any sense to me ... that got me emotionally," Mullins-Silverstein said. "I don't think it would go backwards at all."
He said "ridiculous" claims that "Oh well, the Scots, they take more than they give" to the U.K. in an economic sense "really incense me."
The only answer, he decided after just a few weeks of his internship, was independence.
"It just made sense to me. It was kind of one of those 'duh' issues," Mullins-Silverstein said.
A referendum in 1997 ended with Scotland being given its own parliament in Edinburgh. It has control over domestic spending but only limited powers to change tax rates.
But Mullins-Silverstein said this was designed to placate nationalist feelings and maintain the union with England, and "not to make things better" for the people of Scotland. It would never be allowed to take the really important political decisions, he said.
"They are never going to give the 'biggies' to Scotland," he said. "People stiffen up when you talk about it because they are afraid of losing powers from the center."
Few would admit it, but some Nationalists in Scotland are motivated by romantic notions about the country's past.
But for Mullins-Silverstein "that romantic idea obviously doesn't resonate as much, because I'm not from here." Instead, his commitment to the cause stems from a strong belief in a simple concept: "fairness."
He regularly finds himself arguing the case for independence to native Scots, who either oppose the idea or remain to be convinced.
Getting advice on such an emotive subject from a foreigner has never provoked a hostile reaction in Scotland, he said, but it had occurred once.
"That only ever happened in Florida," where he worked in the state senate before coming back to the U.K. in 2009, he said.
A Scot who had lived in the U.S. for some years got "a bit upset with me," Mullins-Silverstein said, and the man angrily asked "What do you know about Scotland?"
Since his return, Mullins-Silverstein has worked for Angus MacNeil, the SNP lawmaker in the London parliament who represents the remote Outer Hebrides islands.
The Outer Hebrides, off the north-west coast of mainland Scotland, are very different in many ways to the rest of the country: The islands are among the few remaining places where the Gaelic language is widely spoken and where attendance at one of the many churches is the norm. On the main island of Lewis, most shops do not open on a Sunday out of respect for the Sabbath.
The islands' capital, Stornoway, is the only sizeable town. Much of Lewis is made up of a giant peat moor, beautiful in a windswept, desolate way that has been the inspiration for Gaelic poets.
And while achieving independence would mean Mullins-Silverstein would lose his job — as there would not be any Scottish lawmakers in the London parliament — he potentially could gain a new home.
"After the vote, I'd like to move up to Scotland. Edinburgh is great. I just like walking around there. If I could live in Edinburgh right now, then I would. I'd love to live in Stornoway as well," he said.
"I'd like to settle somewhere. I've been kind of a nomad for the past few years. I don't know if I feel Scottish or not, but I find myself using a lot of Scottish terms. Right now I feel like me. We'll see what happens."