My purpose in this essay is to explain and discuss the importance of the “moral twin earth” argument. This theory focuses on the argument first proposed by Hare, that to have genuine moral disagreement requires a common understanding of the concept in question. This theory is important because it openly refutes Cornell's realism, which denies that moral terms, such as "law", are synonymous with any non-moral terms that may be part of the properties identified by "law"; properties related to moral terms, such as "right", are responsible for their use. Therefore, the goal is to show how the “moral twin earth” argument undermines the idea that moral terms refer to some natural properties of actions, which in turn govern our moral practices. The “moral twin earth” thought experiment says that we must imagine that all uses of the word "good" ("good-E") by human species on Earth imply the natural property, which makes things "good" according to a normative consequentialist theory, say the happiness maximization theory. The terrestrial human species would adopt any moral theory that allowed happiness to be maximized as beneficence. Next, imagine that in a separate galaxy there exists an earth, the Moral Twin Earth. Moral Twin Earth is so close to being identical to Earth; Twin Earth species speak a language that is linguistically and semantically identical to that spoken by terrestrial species, English. Furthermore, and most importantly, moral terms like “good,” “bad,” “right,” and “wrong” have the same functional properties as they do on Earth. (Horgan & Timmons 1992) Twins use these moral terms to understand certain actions. For example, they try to do things that they believe are "good" for; and the... center of the paper... let's go back to the Open Question Topic again. The moral terms "good" on Earth and Twin Earth will be governed by a certain natural property, but one can always question whether that term is actually good. It's an open question whether maximizing happiness is good or whether maximizing happiness is good. If the identity of properties does not require synonymy, then it is fair to say that moral terms fail to refer to natural properties. The reference of some terms is not determined by a description or concept with which they are associated. Works Cited Horgan, T., & Timmons, M. (1992). Problems on the Moral Twin Earth: Moral Queerness Revived. Synthesis, 2, 221-260.Merli, David., (2002). Return to the Moral Twin Earth. Canadian Journal of Philosophy,32, 207-240.Rubin, M. (2013). Gritting your teeth on Twin Earth Morality. The University of Western Australia, Australia.