The article discusses the concept of "demand price" in economics, which was introduced by Alfred Marshall in 1890. It refers to the price a consumer is willing to pay for an additional unit of a product, based on how much they already have. The more they have, the lower the price they are willing to pay for more. This idea is represented by a demand schedule or curve, showing how price changes with quantity. For products that can be divided into smaller units, the demand price is the price per unit a consumer is willing to pay for a tiny increase in quantity.