 If \(\phi\) denotes an unknown function, and \(\psi_1,\ldots,\psi_k\) are known functions, and if the \(\psi\)’s and \(\phi\) are substituted in one another in the most general fashions and certain pairs of the resulting expressions are equated, then, if the resulting set of functional equations has one and only one solution for \(\phi\), \(\phi\) is a recursive function. (Gödel 1934, 308)