The relationship between the complexity classes P and NP is a question that has not yet been answered by the Theory of Computation. The existence of a language in NP, proven not to belong to P, is sufficient evidence to establish the separation of P from NP. If a language is not recursive, it can't belong to the complexity class NP. We find a problem in NP which is not in P; because if it would be present in that class, then it will imply that some undecidable problem will be in NP too. That's why it can be confirmed by reduction ad absurdum the following result: P doesn't equal NP. This new problem named Certifying is to find a possible input given a particular deterministic Turing machine named Certified Turing machine and its output.