 More telling is the concern that simplicity is language-relative, and indeed that any theory can be given the simplest specification possible: simply abbreviate it as \(T\)! Lewis replies that a theory’s simplicity must be judged according to its specification in a canonical language, in which all of the predicates correspond to natural properties. Thus, ‘green’ may well be eligible, but ‘grue’ surely is not. (See Goodman 1955.) Our abbreviation, then, has to be unpacked in terms of such a language, in which its true complexity will be revealed. But this now involves a substantial metaphysical commitment to a distinction between natural and unnatural properties, one that various empiricists (e.g. van Fraassen 1989) find objectionable.