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 Historical content is a manifestation of historical thinking.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. There is no way to figure out something without thinking. There is no way to learn how to figure something out without learning how to think it through. There is no way to learn mathematical content without learning how to figure out correct answers to mathematical questions and problems. There is no way to learn historical content without learning how to figure out correct or reasonable answers to historical questions and problems.