Received: 30 April 2018 Revised: 6 September 2018 Accepted: 13 November 2018 DOI: 10.1111/mila.12234 ORIGINAL ARTICLE Water is and is not H2O Kevin P. Tobia1 | George E. Newman2 | Joshua Knobe3 1Department of Philosophy, Yale University, New Haven, Connecticut The Twin Earth thought experiment invites us to consider 2School of Management, Yale University, a liquid that has all of the superficial properties associated New Haven, Connecticut with water (clear, potable, etc.) but has entirely different 3 Program in Cognitive Science and deeper causal properties (composed of “XYZ” rather than Department of Philosophy, Yale University, New Haven, Connecticut of H2O). Debates about natural kind concepts have sought to accommodate an apparent fact about ordinary people's Correspondence judgments: Intuitively, the Twin Earth liquid is not water. Kevin P. Tobia, Department of Philosophy, Yale University, New Haven, CT 06511. We present results showing that people do not have this Email:
[email protected] intuition. Instead, people tend to judge that there is a sense in which the liquid is not water but also a sense in which it is water. KEYWORDS concepts, dual character, experimental philosophy, natural kinds, Twin Earth 1 | INTRODUCTION Imagine a liquid that is exactly like H2O in its superficial properties. It has the same color, texture, and taste of H2O and it quenches thirst in just the same way. However, this liquid is completely dif- ferent when it comes to its deeper causal properties. When chemists examine it, they find that it is not composed of H2O but of some very different chemical compound. Is this liquid water? This is the well-known “Twin Earth” thought experiment.