Implicit atheism and explicit atheism are subcategories of atheism coined by George H. Smith (1979, p.13-18). Implicit atheism is defined by Smith as "the absence of theistic belief without a conscious rejection of it" (i.e., those who have not thought about the existence of deities, let alone decided against it, are de facto atheists). Explicit atheism is defined as "the absence of theistic belief due to a conscious rejection of it" (those who have thought about the existence of deities and have concluded they do not believe any exist), which, according to Smith, is sometimes characterized as antitheism.