The formal theory of computability, providing a systematic answer to the question "what (and how) can be computed?". Formulas represent interactive computational problems, logical operators stand for operations on problems, and validity means being "always computable". Introduced by Japaridze (http://www.csc.villanova.edu/~japaridz/) in 2003. External web source: Computability Logic (http://www.cis.upenn.edu/~giorgi/cl.html).