Cobham began by citing the evidence motivating the Invariance Thesis as suggesting that the question of whether a problem admits a polynomial time algorithm is independent of which model of computation is used to measure time complexity across a broad class of alternatives. He additionally presented a machine-independent characterization of the class \(\textbf{FP}\) – i.e. functions \(f:\mathbb{N}^k \rightarrow \mathbb{N}\) which are computable in polynomial time – in terms of a restricted form of primitive recursive definition known as bounded recursion on notation (see   Section 4.5).