"The question is not about the 'right' or 'best' definition of the word 'racism.' Words are servants, not masters. The real problem is to avoid shifting definitions that play havoc with reasoning."
-- Thomas Sowell, The Economics and Politics of Race, p. 144
I believe there are many examples of shifting definitions over time in public policy discussions that play havoc with reasoning. One might be what happens with the idea of a negative externality. This concept has been precisely defined to be a source of market failure. After some time it seems that using this concept in discussions of public policy choices has led to definitions of negative externality that plays havoc with reasoning. Reasoning has been so distorted or misguided in some cases that negative externality has been used by some to characterize government failure rather than market failure.