Animal people are usually confident that Cartesianism is something of the past and that modern science clearly establishes that animals are sentient beings. But actually the scientific status of sentience is anything but firmly established. Not only is the subjective point of view absent from current science; it is precluded by construction from our fundamental realms of knowledge. Physics — the mother-science once we reject Cartesian dualism — is currently unable to include sentience in its account of the world. A large part of the philosophy of mind describes a mindless mind, from which subjectivity — feeling, qualia — has been stripped, leaving only in place functional relations. This situation paves the way for discourses in which sentience seems to escape the realm of knowledge to fall into that of private beliefs, which individuals can choose as freely as their religion. This is a real obstacle to having animal sentience taken seriously; as such it has been largely underestimated. We believe it necessary for the movement for animals to understand that it cannot by-pass the “mind-matter” problem. We must not allow the existence and relevance of animal sentience to be denied in the name of science. One path we are contemplating is a “Declaration on Sentience” in which scientists and other thinkers would subscribe to the following assertion: sentience is an objective reality of the world and belongs to the realm of science. Despite the current intractability of the “mind-matter” problem, we do have something on our side. Although we cannot prove the reality of sentience, we can show it impossible for anyone to disbelieve in it — just as no one can really disbelieve in the reality of the physical world. We thus have enough reasons to reject the main ways in which sentience is denied or dismissed in many realms of current philosophy or science. These reasons are based on our own situation as sentient and deliberative beings.