H. Freudenthal. Educational Studies in Mathematics, 1 (1/2):
3-8(May 1968)Systematization is a great virtue of mathematics, and if possible, the student has to learn this virtue, too. But then I mean the activity of systematizing, not its result. Its result is a system, a beautiful closed system, closed, with no entrance and no exit. In its highest perfection it can even be handled by a machine. But for what can be performed by machines, we need no humans. What humans have to learn is not mathematics as a closed system, but rather as an activity, the process of mathematizing reality and if possible even that of mathematizing mathematics..
P. Thompson, and L. Saldanha. Research companion to the Principles and Standards for School Mathematics, National Council of Teachers of Mathematics, Reston, VA, (2003)