Yeah, and this particular case is even more specific than "make up any concept you like" - it's specifically inventing a new category of number that is the solu

Yeah, and this particular case is even more specific than "make up any concept you like" - it's specifically inventing a new category of number that is the solution to formulas expressed in existing notation. I think I missed a fourth example of the same pattern: fractions. Originally fractions were not considered to be "numbers", they were just a way of comparing numbers with each other. https://nrich.maths.org/2515 https://nrich.maths.org/2515