John Dupré, in a number of papers and mainly in his paper "Natural Kinds and Biological Taxa", has demonstrated that the theory of natural kinds, which many have taken to be established and/or supported by Putnam's Twin Earth thought experiment does not find support in the practice of scientific classification. Avrum Stroll has produced probably the most comprehensive critique of the program of natural kind semantics (both Putnam's and Kripke's) in his book "Sketches of Landscapes". Putnam, who is well known for changing his philosophical positions on a variety of issues, criticized the experiment later in life because it is anti-functionalist.