{As my new e-mail address was refused by the HES server, this   
contribution arrives two days after it was sent}  
  
As a student, I got entangled in reasonings on what came first, changes   
in quantities or in prices. TilI discovered that the concept of 'price' in  
the Marshallian picture is ambiguous.  
  
'Price' may either mean 'price proper', or 'equilibrium price'. Another   
term for the latter may be 'resulting price'.  
  
It helped me, in those days, to see it this way. In a competitive market,  
suppliers and demanders are price takers from a partial viewpoint. In a   
monopolisitic / monopsonistic market they are granted an influence on   
the price and the quantity. But on the social level, viz. when looking at  
unintended market outcomes, the 'equilibrium price' always is   
something given (ex post). So even if market parties have an influence   
on for the instance the price setting proces, the vertical axis helps you  
to read the 'resulting price' as dependent variable.  
  
If this is true, it makes a lot of sense to keep it the way Marshall (or  
others) pictured it.  
  
Menno Rol