Reconstructing mathematics for learning.

D. Hestenes, a physicist famous for his invention of Geometric Algebra, opened his 2002 Oersted Medal Leture with
"Course content is taken [by many] as given, so the research problem is how to teach it most effectively. This approach [...] has produced valuable insights and useful results. However, it ignores the possibility of improving pedagogy by reconstructing course content."
(Second paragraph, emphasis added.)
Here too, the fact that Hestenes' point is still being ignored by mathematicians goes a long way toward explaining the deplorable state Mathematics Education is in.