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Lawrence Moss writes: 
 
>To summarize, what is exogenous and endogenous is imposed on the 
>real world by the human mind. 
 
I agree. And I would add that it depends on the problem at hand. An  
excellent example of a theorist aware of this is Keynes. In Ch. 18 of the  
General Theory, he first re-states his argument by listing the dependent  
and independent variables and what he takes as given ("this does not  
mean that we assume these factors to be constant; but merely that, in  
this place and context, we are not considering or taking into account  
the effects and consequences of changes in them". p. 245) and by  
explaining how they are causally linked. He explains (p. 247):   
 
"The division of the determinants of the economic system into the two  
groups of given factors and independent variables is, of course, quite  
arbitrary from any absolute standpoint. The division must be made  
entirely on the basis of experience, so as to correspond on the one  
hand to the factors in which the changes seem to be so slow or so little  
relevant as to have only a small and comparatively negligible short-term  
influence on our quesitum; and on the other hand to those factors in  
which the changes are found in practice to exercise a dominant  
influence on our quesitum. Our present object is to discover what  
determines at any time the national income of a given economic system  
and .. the amount of its employment; which means in a study so  
complex as economics, in which we cannot hope to make completely  
accurate generalisations, the factors whose changes *mainly* determine  
our quesitum. Our final task might be to select those variables which  
can be deliberately controlled or managed by central authority in the  
kind of system in which we actually live".   
 
Daniele Besomi 
 
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