Theories of metaphor comprehension that rely on category assignment based on common characteristics (Keysar and Glucksberg, 1992), a comparison between characteristics or relationships of source and target (Chiappe and Kennedy, 2001), or a mapping of characteristics or relationships from source to target (Gentner and Bowdle, 2001) are inherently circular, since these comparisons, mappings, or categories usually make sense only after the underlying metaphor has already been understood. This inherent circularity is avoided by approaches such as conceptual metaphor theory (Lakoff and Johnson, 1980) and conceptual blending theory (Fauconnier and Turner, 1998) that seek to explain metaphors in terms of underlying cognitive concepts or schemas. (David Ritchie)