  So one point is that verification and validation are not independently-successful and separable activities. But the other point is that there are not two independent entities onto which these activities can be directed: a model chosen to discretized, and a method for discretizing it. Once one recognizes that the equations to be “solved” are sometimes chosen so as to cancel out discretization errors, etc. (Lenhard 2007 has a very nice example of this involving the Arakawa operator), this later distinction gets harder to maintain. So success is achieved in simulation with a kind of back-and-forth, trial-and-error, piecemeal adjustment between model and method of calculation. And when this is the case, it is hard even to know what it means to say that a simulation is separately verified and validated.