Thomas Kuhn (1970) observed that science does not progress linearly through a gradual extension of knowledge within one framework; his proposition was a progression through periodic revolutionary upheavals which he called paradigm shifts. He suggested that the discovery of irreconcilable shortcomings in an established framework produces a crisis that may lead to overturning the paradigm and ushering in a new one. Revolutions are never an easy experience and most of us prefer to engage in evolution, where changes happen gradually, often unnoticed, and no-one is unsettled.

http://www.isocarp.net/Data/case_studies/2341.pdf