> Circles and the relationship between their circumference and diameter existed before minds existed

There really aren't any circles in the physical universe. There aren't any ellipses either. I'm not sure about lines. You're going to hit Planck's constant and the speed of light in any example you name.

Charge seems quantized, so you can probably have the integers and fractions, but real numbers are right out.

Bubbles are perfectly circular, and natural.

I mean, yeah, Howard Zinn "told on himself" several hundred pages before that quote from the afterword. His whole thing is that lots of American history is only told from one side and he wanted to popularize a different view. The point of that quote is that not everyone lays their biases out up front the way he did.

In your math example, yes, pi is a constant of nature that simply is. However, in choosing pi over Euler's number or the square root of -1 for your comment, you've made an implicit judgement about their relative importance. That's probably inconsequential for most math facts, but could be a lot more impactful for people presenting history or the news.

Yes. Perhaps Howard Zinn has an agenda.

If you don't like Zinn, how about Popper by way of Deutsch in The Beginning of Everything:

We never know any data before interpreting it through theories. All observations are, as Popper put it, theory-laden, and hence fallible, as all our theories are. . . . The growth of knowledge consists of correcting misconceptions in our theiories.

Pi is a great example. Did circles exist before minds existed? Or are circles approximations of the observed world?

I was thinking about this in the shower the other day. Circles don't "exist" because they are a pure abstraction; as an element of mathematics they are simply a bag of conclusions that are inevitable if you are willing to accept other axioms. The language with which we talk about these axioms and conclusions might be fallible, and certainly the conclusions themselves may be fallible (though not likely so for elemental enough mathematics), but the kernel of truth in logic at the middle is, I think, unassailable.

Perhaps so. I trimmed down the parts where I referenced Euclid in a similar vein to “conclusions … inevitable if you … accept other axioms.” I was thinking though of this Veritasium video outlining the development of non-Euclidean geometries:

https://youtu.be/lFlu60qs7_4?si=vd1s7vPmV-drPSfP