nouns (fields of study) or adjectives. Going by the OED citations, it began being used with specific nouns in connection with mathematical logic sometime before 1929. (In 1920 David Hilbert proposed a research project in what was called "metamathematics.") A notable early citation is W. V. O. Quine's 1937 use of the word "metatheorem", where meta- has the modern meaning of "an X about X". Douglas Hofstadter, in his 1979 book Gödel, Escher, Bach (and in the 1985 sequel, Metamagical Themas), popularized this meaning of the term. The book, which deals with self-reference and strange loops, and touches on Quine and his work, was influential in many computer-related subcultures and may be responsible for the popularity of the prefix, for its use as a solo term, and for the many recent coinages which use it. Hofstadter uses meta as a stand-alone word, as an adjective, and as a directional preposition ("going meta," a term he coins for the old rhetorical trick of taking a debate or analysis to another level of abstraction, as when somebody says "This debate isn't going anywhere"). This book may also be responsible for the association of "meta" with strange loops, as opposed to just abstraction. According to Hofstadter, it is about self-reference, which means a sentence, idea or formula refers to itself. The Merriam-Webster Dictionary describes it as "showing or suggesting an explicit awareness of itself or oneself as a member of its category: cleverly self-referential". The sentence "This sentence contains thirty-six letters," and the sentence which embeds it, are examples of "metasentences" referencing themselves in this way. As maintained in the book Gödel, Escher, Bach, a strange loop is given if different logical statements or theories are put together in contradiction, thus distorting the meaning and generating logical paradoxes. One example is the liar paradox, a paradox in philosophy or logic that arises when a sentence claims its own falsehood (or untruth); for instance: "This sentence is not true." Until the beginning of the 20th century, this kind of paradox was a considerable problem for a philosophical theory of truth. Alfred Tarski solved this difficulty by proving that such paradoxes do not exist with a consistent separation of object language and metalanguage. "For every formalized language, a formally correct and factually applicable definition of the true statement can be constructed in the metalanguage with the sole help of expressions of a general-logical character, expressions