According to English political philosopher John Gray, the divide between human silence and the silence of animals hinges on the use of language. This does not mean that animals lack language, however, since the speech of the birds is much more than human linguistic metaphor. Rather, since animals do not live in their sleep, or speak to themselves when they go about their daily business, the distinction is that humans are the only species whose members use words to construct images of the self and stories of their lives. While other animals lack this inner monologue, Gray maintains that is not clear why this feature should put humans above them.