You and I have discussed the axiom of infinity in this thread. Perhaps I’m misremembering. If we have discussed the axiom of infinity, then your remark is disingenuous. If I’m mistaken and we haven’t discussed the axiom of infinity, I’ll try to remember that I’m talking to a bunch of 5 year olds. That actually explains a lot.
That is a very interesting remark. Of course if you took freshman calculus, you can do that using a rote procedure, say by taking an antiderivative of the kind of elementary functions you see in calculus class. Integrand is (x^2) so antiderivative is (\frac{1}{3} x^3) kind of thing.
But if you studied the subject more deeply, you would realize that in order to form a logically rigorous definition of an integral, you require modern infinitary set theory. In calculus they don’t show you that. Perhaps you remember that when they defined the Riemann integral, they defined lower and upper sums relative to a partition, and then you took the LIMIT over all possible partitions. To formalize that requires the full apparatus of ZF set theory, including the axiom of infinity.
So to me, the fact that you DO believe in Riemann integration (aka freshman calculus integration) tells me that you’ve seen the rote procedures, but not the underlying theory nor all the weird counterexamples and corner cases that made 19th century mathematicians realize they needed a rigorous theory. Infinitary math is essential to define an integral and do freshman calculus. They just don’t tell you about this until you take a more advanced course in real analysis.
No infinitary math, no logical foundation for freshman calculus. No axiom of infinity, no Riemann integral.
ps – Let me give a concrete example. You mentioned integrating an area over a height. How about if you have a rectangular metal plate with a temperature at each point and you want to integrate the temperature over the area of the rectangle to determine the average temperature. You could integrate the vertical slices then the horizontal ones or vice versa. This is multiple integration as in second year calculus. But how do you know when the order of integration matters and when it doesn’t? How do you know whether it makes a difference if you integrate the x’s and then the y’s, or first the y’s and then the x’s? This can be a very tricky business, especially with a weird or pathological integrand or temperature function. This is when you have to drill down to the rigorous, set-theoretic definition of the integral to prove theorems on reversing the order of integration. In other words the moment you go beyond the simplest examples you need some theory; and the theory of integration requires infinitary set theory, or my name’s not Guido Fubini!
en.wikipedia.org/wiki/Fubini%27s_theorem
The end of the Wiki article gives specific examples where reversing the order of integration gives a different answer.