In this paper the writer attempts to derive and apply a theory of validity for inductive or probable argumentation. The following definition of rhetorical validity is proposed: an argument is valid when, in an adversarial situation, the degree of certainty claimed by (or for) a conclusion is less than or equal to that established by its related supporting proof. This definition is based on the following terms and relationships which are consistent with contemporary usage: claims are modified by implicit or explicit qualifiers, claims are drawn from a context in which both favorable and unfavorable evidence resides, and claims should not be considered as valid if they assert a greater probability than is identifiable via scrutiny of the argumentative context. The paper then seeks to establish both empirical and critical procedures for aPplying validity tests to the wide spectrum of nondeductive arguments. A list of references is included. (JM)