 Because the mechanism, as computationally characterized, would not track these [external] properties in every environment, the semantic interpretation of the device is not an essential characterization, and cannot serve to individuate it. However, the semantic interpretation enables us to specify the cognitive function of the mechanism, to characterize it as computing [visual] depth from disparity, for example, or as computing the syntactic structure of a sentence. (Egan 2014: 98)