A paradigm shift (or revolutionary science) is, according to Thomas Kuhn, in his influential book The Structure of Scientific Revolutions (1962), a change in the basic assumptions, or paradigms, within the ruling theory of science. It is in contrast to his idea of normal science. According to Kuhn, “A paradigm is what members of a scientific community, and they alone, share” (The Essential Tension, 1977). Unlike a normal scientist, Kuhn held, “a student in the humanities has constantly before him a number of competing and incommensurable solutions to these problems, solutions that he must ultimately examine for himself” (The Structure of Scientific Revolutions). Once a paradigm shift is complete, a scientist cannot, for example, reject the germ theory of disease to posit the possibility that miasma causes disease or reject modern physics and optics to posit that aether carries light. In contrast, a critic in the humanities can choose to adopt an array of stances (e.g., Marxist criticism, Freudian criticism, Deconstruction, 19th-century-style literary criticism), which may be more or less fashionable during any given period but all regarded as legitimate. Since the 1960s, the term has also been used in numerous non-scientific contexts to describe a profound change in a fundamental model or perception of events, even though Kuhn himself restricted the use of the term to the hard sciences. Compare as a structured form of Zeitgeist.