The argument from species overlap has been widely used in the literature on animal ethics and speciesism. However, there has been much confusion regarding what the argument proves and what it does not prove, and regarding the views it challenges. This article intends to clarify these confusions, and to show that the name most often used for this argument (‘the argument from marginal cases’) reflects and reinforces these misunderstandings. The article claims that the argument questions not only those defences of anthropocentrism that appeal to capacities believed to be typically human, but also those that appeal to special relations between humans. This means the scope of the argument is far wider than has been thought thus far. Finally, the article claims that, even if the argument cannot prove by itself that we should not disregard the interests of nonhuman animals, it provides us with strong reasons to do so, since the argument does prove that no defence of anthropocentrism appealing to non-definitional and testable criteria succeeds.