 Though Duhem embedded this claim for continuity in theory-change in an instrumentalist account of scientific theories, he also took it that science aims at a natural classification of the phenomena, where a classification (that is the representation of the phenomena within a mathematical system) is natural if the relations it establishes among the phenomena gathered by experiments “correspond to real relations among things” (1906 [1954: 26–27]). Hence, scientific knowledge does go beyond the phenomena but in doing so, that is, in tending to be a natural classification, it can extend only up to relations among “hidden realities whose essence cannot be grasped” (1906 [1954: 297]). A clear mark of the naturalness of a classification is when it issues in novel predictions (1906 [1954: 28]). Hence, successful novel predictions issued by a theory are a mark for the theory getting some aspects of reality right, viz. real relations among unobservable  entities.[2]