Kuhn argues that scientific progress is not always linear or developmentally progressive.  Sometimes a completely new paradigm sweeps away formerly accepted "facts and theories".  Our understanding of the universe, for example, was turned on its head when the Ptolemaic view, or paradigm, which assumed the earth was central and all else revolved around it, was overthrown by the Copernican paradigm which recognized the centrality of the sun.  By extension, behaviorists have argued that traditional mentalistic concepts in psychology represent a paradigm that has been upended by a greater understanding of the control the environment exerts upon us.  If you've ever wonder how the word "paradigm" became a central part of the vernacular of every scientist, look no further. 
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