The integrity of professional service in psychology depends upon its scientific foundation. Continual strengthening of that foundation is therefore essential. The complexities of many conditions that engage practitioners, however, limit the extent to which practice can be based exclusively on scientific knowledge and still provide maximum benefit to the public. Resolution of this dilemma can be advanced by adopting a strong definition of science but acknowledging that some problems confronting practitioners are inaccessible to rigorous scientific inquiry and require alternative, primarily humanistic approaches. Three levels of pragmatically acceptable scientistic confidence are defined, the advantages of constructive dialogue across these levels are discussed, and the view of psychology as a pluralistic rather than monistic discipline is advocated.