In Western European political science, the term polyarchy poly 'many', arkhe 'rule') was used by Robert Dahl to describe a form of government in which power is invested in multiple people. It takes the form of neither a dictatorship nor a democracy. This form of government was first implemented in the United States and France and was gradually adopted by many other countries. According to Dahl, the fundamental democratic principle is “the continuing responsiveness of the government to the preferences of its citizens, considered as political equals” with unimpaired opportunities. A polyarchy is a state that has certain procedures that are necessary conditions for following the democratic principle. In Western European political science, the term polyarchy poly 'many', arkhe 'rule') was used by Robert Dahl to describe a form of government in which power is invested in multiple people. It takes the form of neither a dictatorship nor a democracy. This form of government was first implemented in the United States and France and was gradually adopted by many other countries. According to Dahl, the fundamental democratic principle is “the continuing responsiveness of the government to the preferences of its citizens, considered as political equals” with unimpaired opportunities. A polyarchy is a state that has certain procedures that are necessary conditions for following the democratic principle. In semblance, the word polycracy describes the same form of government, although from a slightly different premise: a polycracy is a state ruled by more than one person, as opposed to a monocracy. The word is derived from Greek poly which means 'many' and kratos which means 'rule' or 'strength.' Dahl's original theory of polyarchal democracy is in his 1956 book, A Preface to Democratic Theory. His theory evolved over the decades, and the description in later writings is somewhat different.