Is Anything Truly Impossible?

"People who know what they're doing know the rules, and they know what is possible and impossible. You do not, and you should not. The rules on what is possible and impossible were made by people who have not tested the bounds of the possible by going beyond them."

This is a quote, taken from Neil Gaiman's commencement speech at the University of the Arts, that I found to be very inspirational and that I think underlies the basis of human advancement. After all, who is to say what is possible or impossible if, every day, we learn how to do things that, maybe a century ago, were deemed impossible. Had Galileo conformed to the scientific standards of his day, which stated that the sun and all other planetary bodies revolved around the Earth, would we have ever discovered that the Earth in fact, orbits around the sun? Or, had Goddard not invented the multistage rocket in 1914, would we still believe that space travel is impossible? These are two examples of human achievement that may not have been possible if the individuals had not tested the bounds of the impossible. Can any of you come up with any?