Welcome to Secret Philly, an occasional series in which Billy Penn will visit hidden or exclusive places in Philadelphia and write about them.

Just around the corner from Independence Hall on 5th Street is a much less assuming structure. Separated by just a few feet from the back of the Hall, this fading brick building that’s the only private structure on Independence National Historical Park sometimes gets extra foot traffic from Independence Hall — people stumbling in thinking it’s part of the experience.

But this separate building holds stories of its own that give a glimpse into some of the most brilliant minds, not of our time, but ever.

Philosophical Hall and three buildings near it are the campus of the American Philosophical Society, the oldest still-standing group of its kind in the nation and one that pre-dates America itself. Founded in 1743 by Benjamin Franklin as a way to bring together people to “improve the common stock of knowledge,” the American Philosophical Society is, interestingly enough, more than just philosophers.

Since it was established, members have been leaders in their own disciplines, whether it’s science, humanities, the arts, history, medicine, or other fields. The first three presidents of the organization were Franklin, David Rittenhouse (yes, that one) and Thomas Jefferson. Other colonial members were some of the leaders of the Revolutionary period, including Thomas Paine, John Adams, George Washington and Alexander Hamilton.

The Philosophical Society still has annual meetings that aim to forward the pursuit of knowledge. It has had a total of 5,000 members; 1,000 of them are alive today. Some of the modern members you may recognize: Former president Jimmy Carter, actor and film director Martin Scorcese, the late astronaut Neil Armstrong, renowned cellist Yo-Yo Ma, investor Warren Buffett, the late journalist Walter Cronkite, and the Notorious RBG — Supreme Court Justice Ruth Bader Ginsburg. And today, one in every 10 members of the society is the owner of a Nobel prize.

Many of the members, certainly the colonial ones, walked the halls of Philosophical Hall on 5th Street. Billy Penn went inside to see some of the history that’s still there:

The APS was founded in the 1740s by Franklin who modeled it off similar groups in England — he wanted it to be a “learned society,” a much broader definition than what philosophy means today. The society took off in the 1760s, and Philosophical Hall was built in 1789.

It was founded as a “members only” society and an exclusive group, but it’s always been a collecting institution that saves books, manuscripts and historical relics that give a glimpse into the many accomplishments of the society’s members. Among the collections in Library Hall of the APS are a number of manuscripts — many of which can’t be pulled out or see the light of day for preservation’s sake — including a rough draft of the Declaration of Independence, complete with writing in the margins.

Each year, APS displays a different theme of its collection in Philosophical Hall, and this year’s collection pays homage to Jefferson, who was president of APS for 18 years before, during and after his presidency, according to Curator of Museum Education Gigi Naglak, who took us around on a tour this week.