Some philosophers deny that animals warrant direct moral concern due to religious or philosophical theories of the nature of the world and the proper place of its inhabitants. One of the earliest and clearest expressions of this kind of view comes to us from Aristotle (384-322 B.C.E.). According to Aristotle, there is a natural hierarchy of living beings. The different levels are determined by the abilities present in the beings due to their natures. While plants, animals, and human beings are all capable of taking in nutrition and growing, only animals and human beings are capable of conscious experience. This means that plants, being inferior to animals and human beings, have the function of serving the needs of animals and human beings. Likewise, human beings are superior to animals because human beings have the capacity for using reason to guide their conduct, while animals lack this ability and must instead rely on instinct. It follows, therefore, that the function of animals is to serve the needs of human beings. This, according to Aristotle, is “natural and expedient” (Regan and Singer, 1989: 4-5).
The usual manner of justifying the claim that animals are not equal to human beings is to point out that only humans have some property, and then argue that that property is what confers a full and equal moral status to human beings. Some philosophers have used the following claims on this strategy: (1) only human beings have rights; (2) only human beings are rational, autonomous, and self-conscious; (3) only human beings are able to act morally; and (4) only human beings are part of the moral community.
i. Only Human Beings Have Rights
On one common understanding of rights, only human beings have rights. On this conception of rights, if a being has a right then others have a duty to refrain from infringing that right; rights entail duties. An individual that has a right to something must be able to claim that thing