I specifically said "_among_ the most intellectually demanding". Also, I was just listing too commonly known fields that are both difficult and economically use

I specifically said "_among_ the most intellectually demanding". Also, I was just listing too commonly known fields that are both difficult and economically useless. There are many other fields with problems as difficult as math/physics, but those like biology are economically useful. Which is why public+private funding for biology is, rightfully, many times that of fundamental (not applied) physics/math.