The first and most important insight necessary for the appropriate design of instruction and curriculum is that content is, in the last analysis, nothing more nor less than a mode of thinking. Let me explain.
Historical content is a manifestation of historical thinking.
Biological content is a manifestation of biological thinking. Algebraic content is a manifestation of algebraic thinking. There are many ways to begin to grasp the profound truth that all content is nothing more nor less than a mode of thinking, a way of figuring something out, a way of understanding something through thought. All subjects are areas of study. All fields of study have been advanced insofar as we have discovered ways to figure out whatever is being studied.