 Even more strikingly, Hempel (1988a,b) observed that the application of scientific theories presupposes the absence of factors that might affect the internal principles of the theory, which goes beyond the content of the theory itself. Deriving predictions and explanations from classical mechanics presupposes that bodies are being acted upon exclusively by gravitational forces, for example, where the presence of electromagnetic forces would invalidate those derivations. This is not simply a matter of conducting tests “under suitable conditions”, where the malleability of gold differs when struck by a hammer when at extremely cold temperatures (where the combustibility of paper differs when the paper is wet, and so on), which are familiar examples of the various specific conditions that must be identified in complete definitions of dispositional predicates. What Hempel noticed is that these properties may not only be affected by conditions covered by the theory being applied but involve entirely different theories. He took this to mean that the application of theories has to be accompanied by “provisos” affirming that no properties beyond those specified by the theory are present in a specific case.