“Quantum mechanics is what you would inevitably come up with if you started from probability theory, and then said, let's try to generalize it so that the numbers we used to call "probabilities" can be negative numbers. As such, the theory could have been invented by mathematicians in the 19th century without any input from experiment. It wasn't, but it could have been.”[1]

Still a bit too late for Leibniz by that argument, but within the ballpark.

[1]: http://www.scottaaronson.com/democritus/lec9.html

I could have sworn he wrote a bunch of things late in his life that, when finally published with the rest of his work in the late 20th century, revealed more quantum theory related philosophy.

Unfortunately, I don't have my notes from when I was researching this a few years ago, so I will have to concede that this may be faulty memory attributing things to the wrong individual.