Heisenberg's uncertainty principle
The practical result of this mathematical discovery is that when a physicist makes position more clear then momentum becomes less clear, and that when the physicist makes momentum more clear then position becomes less clear. Heisenberg said that things are "indeterminate," and other people liked to say that they were "uncertain." But the mathematics shows that it is the things in the world that are indeterminate or "fuzzy," and not that it is just that humans are uncertain about what is going on.