Is Calculus a Fiction?

In your book, I really like this line you wrote. What fascinates me as an applied mathematician is the push and pull between the real world around us and the ideal world in our heads. Phenomena out there guide the mathematical questions we ask. Conversely, the math we imagine sometimes foreshadows what actually happens out there in reality. And later you wrote, it's eerie that calculus can mimic nature so well. Can you elaborate on that a little bit? Why do he think math is so good at describing the universe if it's just sort of a fiction in our minds?"