Roger Sandilands wrote:  
  
Young made these comments as a prelude to a long discourse on  
"(i) the limitations of Marshall's one-thing-at-a-time approach to supply and demand
curves;"
  
This  creates more problems for average principles students than many other abstractions
of economics. Most of them tolerate it and move on to other disciplines.
  
"(ii) why a demand curve is not a utility curve, or, at least, why Marshallian consumer
supluses cannot be added to give a measure of total welfare;  "
  
Introductory textbooks in fact treat demand curves as strictly utility curves and they end
up seeming extremely hypothetical. When students bring in the supply and cost factors in
their frame of reference, they have to be quickly reminded of the separation of the two
for analytical reasons.
  
"On Marshall's curve, demand equals the amount that will be purchased at a given price,
and really means hypothetical purchases. Movement up and down the curve does not mean a
change in demand. While the older economists were talking about changes in demand, with
Marshall this can only be shown by shifting the whole curve..."
  
It is a neat technique, but another hurdle for majority of the students. Today's news is
that finally increasing gasoline prices have started to have an effect of declining
'demand'. I will have to explain to the students that the reporters and writers have
either forgotten their econ 101 or they never really got it.
  
Gary Mongiovi wrote:  
   
"I  raise the issue myself in my principles classes, because I think the reversal of the
axes is one of the things that throws students off balance (unconsciously perhaps) when
they're trying to assimilate an unfamiliar mode of thinking. Best to get it out on the
table and talk about it up front.
  
I go through the business of Marshall's Principles locking in the reversal and I talk
about how he approached the construction of the demand function: Instead of asking, as
modern economists do, "how much can be sold at such & such a price?" he came at it by
asking "if I wish to sell such & such an output, what price can I charge?", which makes
quantity the independent variable. I also make the point that there's not much difference
in the two ways of posing the question. "
   
I feel that the two questions still represent the seller's perspective and the consumer's
hypothetical demand curve is simply assumed. And Gary, if your students understand your
skillful explanation, I will make a deal with you. Let us exchange two of my students for
one of yours.
  
Sumitra