What Tony says seems reasonable enough but I am tempted to go further.  The context of
ideas is not just other ideas, but also (even more?) the material conditions and lived
experience of the time.  "Practical experience is not merely the ultimate test of ideas;
it is also the ultimate source."
  
That means that the history of economic thought can be viewed as the record of human
attempts to figure out the economic world in which they find themselves, a world moreover
that in every epoch contains features that seem to be new and that therefore impel
development, not just refinement, of the ideas that came before.
  
This way of thinking about HET sees the field as a bridge between economic history and
economic theory.  We are trying to figure out the world we live in, and the record of past
attempts can help us in our task, not because their world was the same as ours (it
wasn't), but because their problem was the same as ours.
  
There is something slightly surreal about even a proper history of ideas if it remains
disconnected from the history of the times that gave rise to those ideas, no?  But, having
said that, I can see a place for such surrealistic history, as well as other approaches,
within a larger research programme.
  
Perry Mehrling