# Why there is no constraint on "Prover" in definition of $IP$?

According to the definition of $IP$ given in Sanjeev Arora and Barak there is no constraint on the running time of the "Prover" ( when "verifier" sends a message to the "Prover" and expects a message in return from it ). Why is it so ? Also the book says the "Prover" might compute some undecidable function. How is that possible ? Am I missing something trivial ?