If you’re feeling down, David “Big Dave” Sylvester wants to help.

“Free high-fives, free hugs, anything to make you smile,” called out Sylvester, 53, as he stood in front of a Philadelphia park on a brisk Thursday morning.

He held a sign reading, “Hello, my name is Big Dave. BigDaveHugsTheWorld,” which he shifted to his left hand every time he reached up to touch palms with those passing by.

Across 30 minutes, dozens of strangers take Sylvester up on his offer, from police officers to suit-clad business people to students hustling from the subway. All reach a hand to the sky for a golden smack, occasionally to be followed by a deep embrace in Big Dave’s arms.

Over the past 17 years, Sylvester has clasped hands or squeezed arms with what he estimates to be around 250,000 people in 36 countries. He chronicles his journey on his website and social media; on Instagram, his handle is @thehumanhigh5. “A hug and a high five is something we can all do — it’s a moment of power.”

Outside of City Hall, that power manifests itself in the form of instant smiles across faces of those of every age. Often isolated in their own worlds, with headphones hanging down from ears to phone, people stop in their tracks to take in the unexpected nature of Sylvester’s sign and welcoming vibe. Sometimes, the power generates not only a quick grin, but sudden tears, too.

“Often they’ll say little else but ‘I really needed that’ and then keep walking,” he said.

Not everyone, of course, wants to interact with Sylvester. This includes tons of passersby casting skeptical looks, and a security guard who not only declines a high five, but eventually kicks Sylvester out of the park.

For Sylvester, however, the goal is simple: to enhance the world one interaction at a time. Sylvester launched into his continuous hugging and high-five mission after he lost a childhood friend on 9/11.

“I felt helpless, and I simply didn’t want to feel like that anymore,” said Sylvester.

In the aftermath of the terrorist attack on the World Trade Center, he noticed a loss of trust and civility within society that he believes has only continued to grow.

“There’s this stranger-danger ideology,” said Sylvester. “Being a large dude with a sign soliciting hugs makes me an oddity, but a smile is your biggest offense — it disarms people, no matter where you are in the world, and sets the mood to let someone know that you’re not trouble.”

His mission began on a bike trip in 2002, starting in Washington state and taking Sylvester back to his hometown of Philadelphia. It continued as he later traveled across Africa from Cairo to Cape Town, in Asia from Istanbul to Beijing, in Australia from Sydney to Melbourne, and all across the United States.

Sylvester recently visited his 50th and final state, Alaska, where he journeyed to six different cities with his BigDaveHugsTheWorld sign.

“I’m fortunate to work a job where I set my own schedule, but I’m not rich,” says Sylvester, a personal trainer.

As Sylvester sees it, everyone deserves a hug — one of the most basic forms of human contact.

“I’ll hug anyone. I’ve even hugged people wearing Confederate hats, in cities where there aren’t many black people, and we had a good moment,” said Sylvester, who is black. “I’m just trying to open people’s minds, even just a little bit. Change doesn’t happen overnight, but from one good moment a lot can begin to happen.”