  In Quine’s idealized schematization of the relation between theory and evidence, evidence for theory consists of observation categoricals. (These latter can, in turn, be tested by observing or contriving situations in which relevant observation sentences are true.) The case which most obviously poses a potential threat to realism is that of a final global theory, a perfected and completed version of our own. What if there are two or more such theories, equally simple, each of which implies all true observation categoricals? (Note that the theory will not be implied by all the true observation categoricals; apart from other considerations, some sentences of the theory will essentially contain terms which do not occur in observation categoricals.)