Medieval commentaries on Aristotle's passage on natural justice were all based on Latin translations. The thesis of this paper is that Robert Grosseteste, the author of the first non-fragmentary Latin version of the Nicomachean Ethics, altered the passage on natural justice to make it compatible with a significant philosophical claim, namely, that things that are just by nature are principally unchangeable, although change might exceptionally occur among them. In so doing, he employed a two-pronged strategy. First, he adjusted several Aristotelian assertions that clearly supported the idea that natural things are subject to change. Second, he managed to safeguard against rejection a famous claim that Aristotle articulates here on behalf of the Sophists: that everything that is by nature is unchangeable and has the same force everywhere, just as fire burns both here and in Persia (II34b25-26).