If a being suffers, there can be no moral justification for refusing to take that suffering into consideration. No matter what the nature of the being, the principle of equality requires that its suffering be counted equally with the like suffering-in so far as rough comparisons can be made-of any other being. If a being is not capable of suffering, or of experiencing enjoyment or happiness, there is nothing to be taken into account. This is why the limit of sentience (using the term as a convenient, if not strictly accurate, shorthand for the capacity to suffer or experience enjoyment or happiness) is the only defensible boundary of concern for the interests of others. To mark this boundary by some characteristic like intelligence or rationality would be to mark it in an arbitrary way. Why not choose some other characteristic, like skin color?

The racist violates the principle of equality by giving greater weight to the interests of members of his own race, when there is a clash between their interests and the interests of those of another race. Similarly the speciesist allows the interests of his own species to override the greater interests of members of other species.[4] The pattern is the same in each case. Most human beings are speciesists. l shall now very briefly describe some of the practices that show this.

For the great majority of human beings, especially in urban, industrialized societies, the most direct form of contact with members of other species is at mealtimes: we eat them. In doing so we treat them purely as means to our ends. We regard their life and well-being as subordinate to our taste for a particular kind of dish. l say “taste” deliberately-this is purely a matter of pleasing our palate. There can be no defense of eating flesh in terms of satisfying nutritional needs, since it has been established beyond doubt that we could satisfy our need for protein and other essential nutrients far more efficiently with a diet that replaced animal flesh by soy beans, or products derived from soy beans, and other high-protein vegetable products.[5]

It is not merely the act of killing that indicates what we are ready to do to other species in order to gratify our tastes. The suffering we inflict on the animals while they are alive is perhaps an even clearer indication of our speciesism than the fact that we are prepared to kill them.[6] In order to have meat on the table at a price that people can afford, our society tolerates methods of meat production that confine sentient animals in cramped, unsuitable conditions for the entire durations of their lives. Animals are treated like machines that convert fodder into flesh, and any innovation that results in a higher “conversion ratio” is liable to be adopted. As one authority on the subject has said, “cruelty is acknowledged only when profitability ceases.”[7]. . .

Since, as l have said, none of these practices cater for anything more than our pleasures of taste, our practice of rearing and killing other animals in order to eat them is a clear instance of the sacrifice of the most important interests of other beings in order to satisfy trivial interests of our own. To avoid speciesism we must stop this practice, and each of us has a moral obligation to cease supporting the practice. Our custom is all the support that the meat-industry needs. The decision to cease giving it that support may be difficult, but it is no more difficult than it would have been for a white Southerner to go against the traditions of his society and free his slaves: if we do not change our dietary habits, how can we censure those slaveholders who would not change their own way of living?

The same form of discrimination may be observed in the widespread practice of experimenting on other species in order to see if certain substances are safe for human beings, or to test some psychological theory about the effect of severe punishment on learning, or to try out various new compounds just in case something turns up….

Article from Tom Regan and Peter Singer (eds.) ‘Animal Rights and Human Obligations’ (New Jersey, 1989).