The Müller-Lyer figure is a famous optical illusion where we think two line segments have different lengths, although the lengths are in fact equal. If our brains generate optical illusions, we might ask ourselves the question whether moral illusions can exist, how we can detect them and what we can learn from them. In this presentation I argue that speciesism - the difference in moral status between humans and non-human animals - can be seen as an example of a moral illusion, where one intuitive moral judgment is in contradiction with other, stronger intuitions. I present five arguments why the species boundary is not morally relevant, and another five arguments why sentience is morally relevant. Together with anthropological and psychological knowledge, we arrive at a very coherent picture against speciesism. The analogy between this line of reasoning and the method to detect optical illusions is clarified, which implies that speciesism indeed is a moral illusion.