[impendia]

> The point of calculus is...

As a math professor who has taught calculus many times, I'd say there are many different things one could hope to learn from a calculus course. I don't think the subject distills well to a single point.

One unusual feature of calculus is that it's much easier to understand at a non-rigorous level than at a rigorous level. I wouldn't say this is true of all of math. For example, if you want to understand why the quadratic formula is true, an informal explanation and a rigorous proof would amount to approximately the same thing.

But, when teaching or learning calculus, if you're willing to say that "the derivative is the instantaneous rate of change of a function", treat dy/dx as the fraction which it looks like (the chain rule gets a lot easier to explain!), and so on, you can make a lot of progress.

In my opinion, the issue with most calculus books is that they don't commit to a rigorous or to a non-rigorous approach. They are usually organized around a rigorous approach to the subject, but then watered down a lot -- in anticipation that most of the audience won't care about the rigor.

I believe it's best to choose a lane and stick to it. Whether that's rigorous or non-rigorous depends on your tastes and interests as a learner. This book won't be for everybody, but I'd call that a strength rather than a weakness.

[msla]

The rigorous form of the non-rigorous version is non-standard analysis: There really are tiny little numbers we can manipulate algebraically and we don't need the epsilon-delta machinery to do "real math". It's so commonsensical that both Newton and Leibniz invented it in that form before rigor became the fashion, and the textbook "Calculus Made Easy" was doing it that way in 1910, a half-century before Robinson came along and showed us it was rigorous all along.

https://calculusmadeeasy.org/

https://en.wikipedia.org/wiki/Calculus_Made_Easy

[zozbot234]

> The rigorous form of the non-rigorous version is non-standard analysis

This is quite overstated. There are other approaches to infinitesimals such as synthetic differential geometry (SDG aka. smooth infinitesimal analysis) that are probably more intuitive in some ways and less so in others. SDG infinitesimals lose the ordering of hyperreals in non-standard analysis and force you to use some non-classical logic (intuitively, smooth infinitesimals are "neither equal nor non-equal to 0", wherein classical reasoning would conflate every infinitesimal with 0), but in return you gain nilpotency (d^n = 0 for any infinitesimal d) which is often regarded as a desirable feature in informal reasoning.

[ghtbircshotbe]

One of the dangers of a non rigorous approach is not being clear about relative rates. If you're not being precise you're going to confuse people when you say eg that in the limit this triangle is a right triangle. Or look at Taylor's theorem. In different limits you can say a curve is a line, a parabola, a cubic, etc.