A metatheory or meta-theory is a theory whose subject matter is some theory. All fields of research share some meta-theory, regardless whether this is explicit or correct. In a more restricted and specific sense, in mathematics and mathematical logic, metatheory means a mathematical theory about another mathematical theory.
The following is an example of a meta-theoretical statement:[according to whom?]
Any physical theory is always provisional, in the sense that it is only a hypothesis; you can never prove it. No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory. On the other hand, you can disprove a theory by finding even a single observation that disagrees with the predictions of the theory.
Meta-theoretical investigations are generally part of the philosophy of science. Also a metatheory is an object of concern to the area in which the individual theory is conceived.
Examining groups of related theories, a first finding may be to identify classes of theories, thus specifying a taxonomy of theories.
The concept burst upon the scene of 20th-century philosophy as a result of the work of the German mathematician David Hilbert, who in 1905 published a proposal for proof of the consistency and completeness of mathematics, creating the field of metamathematics. His hopes for the success of this proof were dashed by the work of Kurt Gödel, who in 1931, used his incompleteness theorems to prove this goal of consistency and completeness to be unattainable. Nevertheless, his program of unsolved mathematical problems, out of which grew this metamathematical proposal, continued to influence the direction of mathematics for the rest of the 20th century.