Você sabe mesmo do que está falando? (I - Demanda e utilidade)

Então você passou em Economia, graduou-se...mas você entendeu mesmo aquilo? Bem, hoje vou colocar aqui um conceito simples, com uma explicação que parece boba, mas que pega muita gente boa de serviço (obviamente, aluno nenhum meu lerá isto e, claro, eu vou cobrar isso em alguma prova....)

When I ask students taking an exam or quiz to explain why the demand curve is the same as the marginal value curve, most of them think they know the answer--and most of them are wrong. The problem seems to be a confusion based on an imprecise verbal argument. It sounds very simple: "Your demand is how much you demand something, which is the same as how much you value it" or, alternatively, "Your demand is how much you are willing to pay for it, which is how much you value it." But both of those explanations are wrong. Your demand curve shows not how much you demand it but how much of it you demand--a quantity, not an intensity of feeling.

Fonte: http://www.daviddfriedman.com/Academic/Price_Theory/PThy_Chapter_4/PThy_Chapter_4.html