In physics courses, it's pretty clear that perfect balls and frictionless planes aren't actually real. OTOH, I recently had a debate with an economist that nearly insulted me because I doubted that infinite GDP growth was possible in a finite world; you "simply" need to infinitely enhance value of goods and services, which is perfectly possible because it hasn't been proved impossible. Plus it's taught in Econ 101, so how stupid should I be to doubt such a little thing?