 This interpretation covers several aspects—semantic ones (concerning the expressive power of a language), metaphysical ones (states of affairs being of one kind), and methodological ones. Assume that any empirical statement, or proposition, can be expressed in a single language. This language is, on Carnap’s view, a language that employs observational concepts only. This seems to amount to an ontologically neutral form of reductionism. In principle, any reductionist—the idealist as well as the materialist—can subscribe to this thesis. Disagreement will start only when it comes to specifying the ontological status of the kinds designated by this language’s observational expressions. Are they external mind independent properties of objects or are they phenomenal properties directly present in perceptual experience?