If there is such a thing as moral progress, one way it is achieved is by expansion of the moral community—that set of beings whose welfare must be considered in moral deliberations. The earliest moralities were tribal. Often these entailed complex duties or taboos with respect to fellow members of the tribe—but they applied to the tribe only and not to those outside of it. The tribe, in other words, was the moral community. One had to respect one's tribal companions, but it was perfectly acceptable to traverse the hill to the next tribe's village and slaughter, rape or enslave them at will.

A late form of tribal morality finds expression in the Old Testament book of Joshua, which tells how the tribes of Israel conquered the villages of Canaan, putting all the men, women and children to the sword. The narrative never questions the propriety of this unprovoked mayhem. The morality is tribal and the Canaanites, being members of alien tribes, simply don't count.

As tribes coalesced into city-states and city-states into nations and empires, the moral community grew as well, but otherwise things stayed largely the same. At the beginning of the nineteenth century, for example, we find Napoleon winning the near universal admiration of the best minds of his time by leading great armies across Europe and killing hundreds of thousands of people. This bloodbath wasn't regarded as a moral matter, for the morality of the time was nationalistic. The enemies conquered or killed didn't really count, because they weren't members of Napoleon's moral community; they weren't French.

It took two world wars to convince most of us that nationalistic moralities are bigoted nonsense. Most of us now at least mouth the morality of human equality—though others still want to deny women or alien racial groups full moral status. Nevertheless, it has finally become common to admit that all humans—regardless of nationality, race, sex or any other incidental characteristic—are, equally, worthy of moral consideration. That is moral progress.

It is tempting to suppose that the expansion of the moral community to the whole human race has finally brought this progress to its ultimate end. But maybe not. There are, for one thing, people whom most of us still fail to accord full moral status—namely those who will live in the future. We commonly ignore future generations in our moral decision-making, even though our orgies of consumption, pollution and species extinction are doing them obvious and predictable harm. Yet time of birth (whether in the current generation or some future one) is no more relevant to moral status than place of birth—or sex, or race. To enlarge the moral community temporally, to future generations, then, is a form of moral progress that, for the most part, remains unrealized.

And even that may not be the end. Before Darwin, it used to be thought that the human species was absolutely unique—created separately from the rest of the animal kingdom. But we know now that this is false, and that we share a common evolutionary history with every living thing on earth. Why, then, should the boundary of the species homo sapiens be of any greater moral significance than the boundaries of tribes, nations, races, sexes or generations?

Many animals have an emotional life; many can feel pain. Some may be even more sensitive than humans, experiencing forms of pain or emotion more intensely. If it is wrong to cause suffering to humans, why is it not just as wrong to cause the same degree of suffering to members of other species?

When this question is baldly put, many people do unreflectively agree that it is just as wrong. I say "unreflectively," because they seldom realize just what this admission implies. For if we take seriously the belief that animal suffering is wrong, then most of us must significantly change our lives. The largest single cause of animal suffering for which humans are responsible is factory farming. In the big corporate farms which produce much of the meat (most, in fact, of the chicken and pork) currently available on the market, tens of billions of sentient animals per year are subjected to a regimen of confinement, overcrowding, unnatural light cycles, hormones, antibiotics, overfeeding, mutilation, and, finally, slaughter. The sole motive for these grimly efficient operations is to produce as much meat as possible as quickly as possible for the least possible dollar investment. What the animals experience is not a consideration. Animals, after all, are not part of the moral community. They don't count.

But if we once recognize that they do count—that, for example, the suffering of a pig or a chicken is just as wrong as the suffering of a human being—then once again, as with nationalism or racism or sexism, our perspective shifts; old ways must be abandoned and new ones made possible. One cannot, for example, continue consistently and in good conscience to consume the products of corporate animal agriculture, since to do so is to not merely to condone but financially to underwrite factory farms. But since more and more of the meat marketed today is produced in industrial operations, the practical upshot is that we must stop—or at least severely curtail—our purchases of meat.

Would that be moral progress? Henry Thoreau thought so—and not merely because of the suffering it would prevent. Relinquishing meat would, he thought, constitute a significant improvement in human character. "It may be vain to ask," Thoreau wrote in Walden,