In Chapter 3 we presented a case for saying that principles of scienti c method are generally of the following sorts: deductive and non-deductive principles of inference; values by which theory choices can be made; and methodological M-principles, which govern theory choice and can include values. On what grounds do we think that any of these are correct? ere is some controversy about the grounds on which the rules of deductive logic are to be justi ed; some argue that they involve circularity since they are employed in their own justi catory arguments (see §6.1). Even more controversial are the grounds for accepting inductive inferences, values and M-principles. It seems appropriate to call the investigation into principles of method and their justi cation metamethodology. is chapter explores what this might be, and what grounds it can o er for accepting principles of scienti c method.