Political movements, to keep moving, need philosophers. If the philosophers are grounded in ethics, the movements they are aligned with have double the force and can't help but spread.

Few current spreadings better illustrate this than the unignorable animal-rights movement and Peter Singer, the Australian philosopher-ethicist. His 1975 book, ''Animal Liberation,'' argued that it is ''wrong to inflict needless suffering on another being, even if that being were not a member of our species.'' In a work that combined scholarship with lithe prose, Singer documented some of the ruthless cruelties inflicted on animals by human beings and put forth persuasive ethical reasons for change.

Tomorrow Singer will be part of the March for Animals here in Washington, a rally at the Ellipse and Capitol endorsed by nearly every U.S. animal-protection group. Except for Noah's genius moment of booking his ark, the March for Animals promises to mark history's most ardent outpouring of collective concern for protecting animals. Like Rachel Carson and ''Silent Spring'' or Ralph Nader and ''Unsafe at Any Speed,'' Peter Singer and ''Animal Liberation'' gave intellectual shape to what was once a formless and obscure expression of dissent. Singer's book took an issue that was on the fringe and put it on the agenda.

In a conversation with him the other afternoon, I found Singer to be a citizen of brave defiance whose ideas are much more than a blind flash of passion without hope of political follow-through. He rebuts the current stereotype of an animal-rights activist as a violent-minded zealot who is either spray-painting fur coats or picketing McDonald's on the evils of eating hamburgers.

In Melbourne, Australia, Singer is professor of philosophy and director of the Center for Human Bioethics at Monash University. He had his wake-up call in England in 1970 while a graduate student at Oxford. Vegetarian friends explained the ethical reasons for avoiding an animal-based diet. ''I became convinced,'' he explained ''that by eating animals I was participating in a systematic form of oppression of other species by my own species.''

In the second and revised edition of ''Animal Liberation,'' which was published this spring as a New York Review book, Singer takes on an issue that hadn't surfaced in 1975 -- activist violence. ''It would be a tragic mistake if even a small section of the animal-liberation movement were to attempt to achieve its objectives by hurting people. Some believe that people who make animals suffer deserve to have suffering inflicted on them. I don't believe in vengeance; but even if I did, it would be a damaging distraction from our task of stopping the suffering. To do that, we must change the minds of reasonable people in our society. We will not accomplish that by violence.''

Before Singer's 1975 book, others had written on behalf of animals, from Henry Salt's minor classic in 1892, ''Animal Rights,'' to Lewis Gompertz's 1824 ''Moral Inquiries on the Situation of Man and Brutes.'' Nor since 1975 has he been alone in the field, sharing it now with Tom Regan, a philosopher at North Carolina State, and with writers for Animals' Agenda magazine. What Singer had done singularly is to give a name -- speciesism -- to the flawed intellectual reasoning that sanctions the slaughter of an estimated 10 million animals a day for food and several million a year by experimenters. Speciesism joins two other scourge "isms" of the 20th century -- racism and sexism -- as unresolved torments imposed by the powerful on the weak.

''Racists,'' Singer writes, ''violate the principle of equality by giving greater weight to the interests of members of their own race when there is a clash between their interests and the interests of those of another race. Sexists violate the principle of equality by favoring the interests of their own sex. Similarly, speciesists allow the interests of their own species to override the greater interests of members of other species. The pattern is identical in each case.''

In the counteroffensive against animal rights -- mounted by the biomedical research lobby, the meat and fur industries and hunting organizations -- no philosopher has come forth to refute Peter Singer. The ethical case for butchery and torture to animals can't be made because none exists.

Speciesists can argue from custom: What's Thanksgiving without a turkey? Or economics: Slaughterhouses provide jobs. Or the Bible: Jesus cast out devils by drowning swine. But habits, profits and scripture aren't ethics. If there's a book to answer Peter Singer -- ''Animal Enslavement'' -- it has yet to be written.