Isaac Newton’s “Philosophiae Naturalis Principia Mathematica,” better known as just “Principia,” is one of the most important scientific works ever written, the source of—among many epiphanies—the physicist’s revolutionary “laws of motion.” And last week, it was the focus of a brazen, two-decade-long theft of rare books from the Carnegie Library of Pittsburgh.

According to the Allegheny County District Attorney’s Office, the library’s former archivist and an antique bookseller allegedly conspired to steal and sell over $8 million worth of rare books from the public library, including “Principia,” valued at $900,000. The archivist, Greg Priore, and the bookseller, John Schulman, were arrested on Friday for the crime.

As Newton’s Third Law goes: for every action, there is an equal and opposite reaction.

Among the other 300-some items stolen from the library’s rare book room were works like “The Journal of Major George Washington” and Adam Smith’s “An Inquiry into the Nature and Causes of the Wealth of Nations.” According to prosecutors, many were stolen in plain sight, with Priore walking them a few blocks over to Schulman’s bookstore after work. Sometimes, he removed pages with an X-acto knife and put the book back on the shelf.

The scheme allegedly began in the late 1990s, when Priore told authorities he began offering to sell books from the closed-to-the-public Oliver Room to Schulman. For his troubles, prosecutors say the bookstore cut Priore $117,700 worth of checks from 2010 to 2017, along with a cash deposit of $17,000 Priore made in the same timeframe.

The missing books and pages came to light in April 2017 after an outside audit of the library, and Priore was fired. An estimated $1.1 million worth of items have been recovered so far, including 42 pieces sitting in Schulman’s Pennsylvania warehouse.