Can
there be other useful assumptions?”2 (Yes, there can be, and yes, there are.)
All key assumptions—in mathematics and every other field—are not somehow
just brilliantly intuited, nor are systems just found, whole, at our feet.
They come from inquiry, over time, based upon a careful search for the logical
grounds of insights we have and the proofs we want to make. Euclid knew that
to prove that there were 180 degrees in all triangles he needed the parallel postulate.
This counterintuitive idea is rarely explained or even adequately suggested
in textbooks. Is it any wonder, then, that many students are confused
about a basic matter—the difference, if any, between axioms and theorems?
Here, then, is another example of what teaching for understanding looks
like: Identify the big ideas and revisit them via problems of increasing sophistication
as the work unfolds, whether or not the textbook does. Don’t “cover”
the big ideas (in this case “axiomatic system”) but rather “uncover” the real
issues lurking below the surface, and keep returning to them, even if the textbook
isn’t organized to do so.