Eliminativism about a class of entities is the view that the class of entities does not exist. For example, materialism tends to be eliminativist about the soul; modern chemists are eliminativist about phlogiston; and modern physicists are eliminativist about the existence of luminiferous aether. Eliminative "materialism" is the relatively new (1960s–1970s) idea that certain classes of mental entities that common sense takes for granted, such as beliefs, desires, and the subjective sensation of pain, do not exist. The most common versions are eliminativism about propositional attitudes, as expressed by Paul and Patricia Churchland, and eliminativism about qualia (subjective interpretations about particular instances of subjective experience), as expressed by Daniel Dennett and Georges Rey. These philosophers often appeal to an introspection illusion.