al econ問題一問?

第一位人兄得罪晒你講錯嘢
嘉芬不是存在於世上，而是嘉芬物品只可以在邏輯上存在，要注意是邏輯上存在，不是真實的存在。為什麼我要再三重申，因為嘉芬物品是指我們隨著價格上升，我們對這種物品嘅需求量不斷上升，價格與需求量有著一種正比例嘅關係。而在正常需求定律下應該是價格與需求量有反比關係。
首先假設嘉芬物品真的存在世上(實際不可能，只是解說需要)，在沒有市場和公司情況下即是以最簡單方法進行交易，一人與一人(Theorem of Exchange)。而嘉芬物品所代表一個人嘅對某種物品需求曲線都是向右上移，這樣出現了一個大問題，兩位交易的人之中，有一位人兄需求曲線向右上移那交易便不能成事。因為需求曲線各右上升那位人兄，老早就將所有物品據伙己有，而一定不會以交易賣掉，你出價愈高他更不願意把物品賣掉。
因此，嘉芬物品只可以在邏輯上存在，因為我們只從個人需求方面看，而忽略了人與人之間的競爭，嘉芬物品根本不可能在市場上成交。即使存在，也只是邏輯上存在於魯賓遜的一人世界中。換言之，人與人之間的競爭淘汰了嘉芬物品。



再回到發問者問題，我們經濟學上在等優曲線分析上或是邊際使用價值分析上也不能引申到需求曲線出來。因為兩種分析也會繪出一條向右上升需求曲線。這兩種分析也會使需求定律有被摧毀嘅可能。但其實是經濟學家不知道嘉芬物品只能在邏輯上存在，因此需求定律仍被假設為正確。

在經濟學家們未知道嘉芬物品只能在邏輯上存在之時為了挽救了需求定律，而出現了有名的"艾智仁通論"(Alchian's Generalization)，艾智仁通論能有效地挽救需求定律，使需求定律仍然存在強大解釋及預測人類行為的能力。

2006-11-21 01:28:42 補充：
Alchian's GeneralizationAlchian's Generalization

第二位人兄: I think he is also asking why indifferent curve CAN DERIVE demand curve.

Let's imagine there is a diagram of indifferent curves and budget line for 2 goods.
A consumer will consume a busket of goods at a point where the budget line is tangent to the indifferent curve in order to maximize his utility. When the the price of one good is changed, the slope of the budget line is changed too. Then, the consumer will choose a new busket of goods (the new tangent point between new budget line and indifferent curve) to maximize his utility.
So, you can see that there is a relationship between the PRICE of a product and the QUANTITY of good which the consumer will buy. Therefore, you can derive a demand curve from the indifferent curve.

Indiffernt curve can derive demand curve with downward sloping and upward sloping, and it DOES NOT IMPLY the demand curve is upward or downward sloping.
The upward sloping demand curve exist in logical sense and so far the economists cannot find a GIFFEN goods in real world.

Anyway, I want to clarify that Law of Demand is an assertion. If you ask me why Law of Demand says there is inverse relationship between price and quantity demand, I will say that it is just human behaviour the eonomists observe.

因為有吉芬物品(giffen good)既存在,令需求定律只係一個假設,不能夠引申