In the previous chapter, we saw that the consumer's demand curve could be traced to the "marginal benefit" the person gets from one more unit of consumption. The "marginal benefit" is the money value of the goods the consumer would give up to get one more unit -- an application of the opportunity cost idea. That's correct and adequate for the purposes of Essential Principles of Economics, but not quite complete in the context of the Rational Dialog that is the history of economic thinking. The idea of "marginal benefit" as the basis of demand has a long history, and the related ideas that have been developed in that history are important in themselves. In the earliest development of the theory of demand, economists tried to tie demand to the one common factor that all goods and services have: utility. This may seem pretty vague, but, surprisingly, we can do a good deal with it. However, there are some criticisms of the utility approach, and many economists prefer to base demand theory on a concept of "preference." The "preference" theory is the one used in more advanced microeconomics courses. This chapter will review the two ideas in turn.