Twardowski See ACT- OBJECT PSYCHOLOGY, POLISH. LOGI. Twin-Earth, a fictitious planet first visited by Hilary Putnam in a thought experiment designed to show, among other things, that ‘ ‘meanings’ just ain’t in the head’ (‘The Meaning of ‘Meaning’,’ 1975). Twin-Earth is exactly like Earth with one notable exception: ponds, rivers, and ice trays on Twin-Earth contain, not H O, but 2 XYZ, a liquid superficially indistinguishable from water but with a different chemical constitution. According to Putnam, although some inhabitants of Twin-Earth closely resemble inhabitants of Earth, ‘water’, when uttered by a Twin-Earthling, does not mean water. Water is H2O, and, on Twin-Earth, the word ‘water’ designates a different substance, XYZ, Twin-water. The moral drawn by Putnam is that the meanings of at least some of our words, and the significance of some of our thoughts, depend, in part, on how things stand outside our heads. Two ‘molecular duplicates,’ two agents with qualitatively similar mental lives, might mean very different things by their utterances and think very different thoughts. Although Twin-Earth has become a popular stopping-off place for philosophers en route to theories of meaning and mental content, others regard Twin-Earth as hopelessly remote, doubting that useful conclusions can be drawn about our Earthly circumstances from research conducted there. See also MEANING, PHILOSO- PHY OF LANGUAG. J.F.H.