Part philosophical, part scientific, Leibniz believed that our world - "the best of all possible worlds" - must be governed by what is known as the Principle of Optimality. This seemingly outlandish idea proved surprisingly powerful and led to one of the most profound ideas in theoretical physics. Jeffrey K. McDonough tells the story.

The great German polymath Gottfried Wilhelm Leibniz famously insisted that ours is the best of all possible worlds. The claim that our world couldn’t possibly be better has never been very plausible. It was hard to believe when Leibniz made it in the seventeenth century on the heels of the horrific Thirty Years War. It didn’t seem any more likely when Voltaire heaped ridicule upon it following the Lisbon Earthquake of 1755. And, of course, it will probably not find many adherents today as we trudge along under the weight of a global pandemic, political uncertainty, and an environment on the verge of collapse. Leibniz’s thought that ours is the best of all possible worlds is, in short, incredible. Incredible or not, however, Leibniz’s implausible idea lies at the heart of one of the most profound, most successful, most tantalizing developments in the theoretical physics. Call it the story of Leibniz’s Principle of Optimality.