We saw in Module 3 that there are two major kinds of change relationships: procedures, which are actions performed to achieve a goal, and principles, which are natural changes, including natural processes and cause-effect relationships. Procedures can be learned rotely. You can learn to follow steps successfully without having any understanding of why you are doing what you are doing, nor why what you are doing works. In fact, it is principles which give one that understanding. Behind every procedure there is at least one principle which explains why it works (although we may not have discovered the principle yet). Hence, it is not procedures which are directly understoodóthey are only understood by understanding the principles which underlie them. 

Let's consider a principle like the law of supply and demand. To understand the relationship between price and the quantity of a good supplied (by producers) and demanded (by consumers) is to form a mental model which integrates it with what you already know. Each such mental model, or cluster of interrelated knowledge, is referred to as a schema.

Understanding is probably the least studied and least understood type of learning within the cognitive domain. Fortunately, it is an area which is currently receiving a lot of attention by learning psychologists.