A mathematician would say “the real numbers are closed under addition, because any real numbers added together always give you another real number”. To “solve” this problem Euler decided to make up a new “number” called “ “, with the property that , and complex numbers were born. Where the closed-ness of real numbers fail, complex numbers hold strong. Finally, here’s the answer, there are a lot of (infinite) number-systems bigger than the complex numbers that contain the complex numbers in the same way that complex numbers contain the real numbers. The smallest number system that’s bigger than the complex numbers is the “quaternions”.