answer:A brief rundown with many omissions: There are two major philosophical schools of moral vegetarianism. The first is the utilitarian school. Utilitarianism is concerned with maximizing happiness for all sentient creatures capable of suffering. On this view, a creature isn’t an object of moral concern until it exists and is sentient. (Note: this is not unique to utilitarianism. Many moral philosophers believe that sentience is a necessary and sufficient condition for being an object of moral concern.) Utilitarians cannot find intrinsic value in the mere existence of a species. If you can show that it significantly undermines the happiness of the remaining species to let one go extinct, then they would say you have a reason to prevent the extinction. Otherwise, future non-existent cows are just as worrisome as all of the siblings I’ll never have because my parents weren’t constantly trying to churn out babies. The second is the deontological school, from which we get the term “animal rights.” Deontological vegetarians argue that the basis of rights is a set of cognitive abilities that are shared by human and non-human animals alike (or at least, many non-human animals). Though non-human animals cannot act as moral agents—that is, they cannot assert their rights, and often lack a deep understanding of ethics—the same can be said of many human animals (infants being the most obvious example). So long as an animal—human or not—is a bearer of rights, it cannot be killed for no reason or owned by anyone else (life, liberty, and self-ownership being considered basic rights that a being must have if they have any at all). But notice again that this is based on individuals. There is no place for attaching value to the species itself, which is not a being (and thus cannot be the bearer of rights).* That the two major schools cannot—or at least do not—place any weight on the species itself has led to at least one other approach, known as deep ecology. This approach rejects all efforts to understand the moral value of animals in terms of the ways in which humans get their value. That is, it takes animals to be valuable in themselves even if they do not share any of the qualities that make humans valuable in themselves. Indeed, deep ecologists typically find moral philosophy to be irredeemably shallow, arguing that it is anthropocentric and based on a fundamental misunderstanding of the world. Ethics, they say, must be holistic. The real question is how to promote the well-being of the system—the planet, or possibly the whole universe—understood as a single functioning unit. Though some beings have no other choice but to prey on other beings, those that can appreciate the intrinsic value each being has are obligated to respect them all and promote their continued existence. —————————— * Note that there is another deontological school that holds closely to Immanuel Kant’s views about animals and what we owe them. According to the Kantians, animals are not morally considerable at all; however, we are required to treat them compassionately in order to prevent ourselves from becoming immoral beings. The idea is that allowing oneself to be cruel to animals will eventually erode our character to the point that we start acting cruelly to other human beings. For whatever reason, Kant didn’t think this prevented us from killing and eating other animals.