Why is it so tough to give a definition for something even if we know what it is? For e.g. Math:- There is no proper definition of math, many people argue about what exactly is it. But still, even if we do not have a proper definition, we know what math is and what it's not. When calculus was invented, everyone knew it was math. So we can tell what is what but can't give a proper definition. Why is that?