Heisenberg's uncertainty principle
There have been two very different ways of looking at what Heisenberg discovered: Some people think that things that happen in nature are "determinate," that is, things happen by a definite rule and if we could know everything we need to know we could always say what will happen next. Other people think that things that happen in nature are guided only by probability, and we can know only how things will behave on the average—but we know that very precisely.