Exchanging goods and services is defining trait of human race. While also some other animals have learned how to harvest nature and craft tools, only members of human species learned how to conduct trade. The essence of trade is exchange of something of a lesser value for something of a higher value, e.g. a farmer with abundance of grain might exchange some of it for a beautiful clay pot. One cannot break a clay pot in smaller units without destroying its value, while grain can be distributed in arbitrary quantities. Consequently, when conducting trade, the potter tries to boast the beauty and versatility of his earthenware while farmer is motivated to dispense as little of his grain as possible while still getting his hands on the pot he wants. The ancient art of haggling has its modern day equivalent in product development. Just like a potter many thousands years ago, a product manager's role is to design a product which will command the highest value for potential buyer. When products were conceived, made, and sold by the same person accounting for all the steps simultaneously could be done inadvertently. But nowadays, when hundreds or thousands of people are engaged with a single product, it's much easier to loose track of the value a product delivers. It is therefore one of the essential roles of a product manager to think about value proposition of the product he or she is building. And it is not just the value of the product for the end user that product manager should be concerned with, but equally important, the perceived value of the product for the buyer at the time of selling, and the total cost (i.e. the negative value) of bringing the product to the market and into the hands of the user.