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The animal rights debate in philosophy has just entered its third decade, if we mark its beginning by the publication of Peter Singer's Animal Liberation. Recently there have emerged lengthy defences of our traditional treatment of animals, for instance, by Peter Carruthers and Michael Leahy, arguing that animal rights need to be challenged in a stronger way than the debate has witnessed.

It is therefore with perfect timing that Evelyn Pluhar brings out her longest and weightiest defence of animal and human rights to date. She aims to refute "unanswered critiques of views that accord moral significance to non human animals" and to establish the rights of "marginal humans" on a secure footing. Marginal humans are humans who are not "full persons", eg the very young and mentally deficient. "Full persons" are those who have certain moral or intellectual abilities, eg the ability to claim rights against others.

Its appreciable detail and scholarship make this book necessary for any thorough understanding of the arguments for and against animal rights, and how these relate to human rights. Yet its structure and clarity make it ideal as an introduction.

Pluhar attacks "species-ism", which states that all and only humans have "full moral significance"; and the "full personhood view", according to which all and only "full persons" do. "Full moral significance" is defined as possessing basic moral rights, including the right to life (utilitarians substitute "strong presumption" for "right").

The book revolves around an "argument from marginal cases", important enough to quote in full:

"1. Beings who are similar in all important morally relevant respects are equally morally significant. 2. Non humans exist who are similar in all important morally relevant respects to marginal humans. 3. Therefore, those non humans who are similar in all important morally relevant respects to marginal humans are maximally morally significant if and only if marginal humans are maximally morally significant."

Pluhar examines no fewer than nine attempts by species-ists to defend marginal humans while not extending moral considerability to animals. Most are alleged to be begging the question, assuming that marginal humans are morally considerable and animals are not. Species-ism emerges as inconsistent.

In the main, she defends the cogency and relevance of the argument from marginal cases very well. But her arguments become tedious and are occasionally glib, which qualities stem from her strategy of "divide and conquer", adopted for the sake of perspicacity.

One response to the argument from marginal cases is based on the utilitarian side-effects of exploiting marginal humans as we do animals, such as fears for our own protection and the emotional attachments we have to some marginal humans. Pluhar claims all this could be "easily circumvented" by secret exploitation and rules protecting full persons who become marginal humans. This is simply naive.

Additionally, Michael Leahy's work, which is the most cogent and challenging to animal rights, seems unfortunately to be unknown to Pluhar.

The full personhood view is not inconsistent so much as frightening. Pluhar emphasises this by falsely (and in contradiction with herself elsewhere) presenting it as defending our current treatment of non-humans, thus allowing such treatment for marginal humans as well. Of course, the theory may argue that marginal humans have some, though not full, moral significance, though if it refuses to attribute equal significance to animals, it must answer to the argument from marginal cases. But it leaves marginal humans without basic moral rights. Most of us choose not to break our teeth on this particular bullet.

After faulting utilitarianism for having a number of unsavoury consequences (nothing ground-breaking here, though it is an excellent discussion), Pluhar outlines her defence of both non human and marginal human life. She defends Alan Gewirth's justification of rights and refers the reader (as I do) to Gewirth's Ethical Rationalism for a fuller discussion. What the argument shows, according to Pluhar, is that all beings who can have conscious purposes they wish to fulfil have basic moral rights. She ends by discussing who such beings are and taking a very balanced and sensitive look at the practical consequences of her position, including cases of conflict.

Pluhar views morality as purely a system of obligations and creates an acknowledged tension between emotion and reason, and theory and practice, in that emotion is external to theory (justification), but internal to practice (motivation). There are good reasons for rejecting this approach, and the development of a more integrated view of morality will open a new era in the debate over the ethical treatment of non-human animals.

Michael Lacewing is freelance tutor in ethics and political philosophy, University of Oxford.