John Heil on "Twin-Earth" in Cambridge Dictionary of Philosophy:
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"... 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.
