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In one paragraph, Mr. DeLong uses three words which betray that which drove (inspirited?) Mr. Friedman.
"His worldview began with a bedrock belief.....
a trust in free markets....
on top of that was layered a powerful conviction...."
'Trust', 'conviction' & 'belief' are keywords and
hallmarks (or warning signs) of the ideologue.
That said...and in substantial but incomplete disagreement with Mr Friedman's thesis...I would wonder if much of economic theory is actually just belief system.
IF that is true, we need more data and less 'religion'...
And will then find actual enlightenment & economic truth.