The following is from History of Economic Analysis. I would like to 
suggest it in place of the last quotation; in fact, it will be a nice 
complement to the quotes mentioned by Dieter Boegenhold. It is 
lengthy, but says something important about the development of economics.

Scientific analysis is not simply a logically consistent process that 
starts with some primitive notions and then adds to the stock in a 
straight-line fashion. It is not simply progressive discovery of an 
objective reality - as is, for example the discovery of the basin of 
Congo. Rather it is an incessant struggle with creations of our own 
and our predecessors' minds and it 'progresses', if at all, in a 
criss-cross fashion, not as logic, but as the impact of new ideas or 
observations or needs, and also as the bents and temperaments of new 
men, dictate. Therefore, any treatise that attempts to render 'the 
present state of science' really renders methods, problems, and 
results that are historically conditioned and are meaningful only 
with reference to the historical background from which they spring. (p. 4).

Sumitra Shah