Reverse mathematics

Reverse mathematics is a way of understanding concepts in mathematics by looking at how difficult they are to prove. It looks at which facts we can easily prove, as well as which facts we need more complicated ideas to prove. For example, if we wanted to prove that two plus two equals four, that would be easy to do. But if we wanted to prove something more tricky like the Pythagorean Theorem, we would need more complicated ideas to prove it.