Jeff McMahan's project is to consider justifications given for the killing and letting die of human beings and animals in certain circumstances. Four of these kinds of circumstances are:
(1) When killing or letting die would promote the greater good, and/or when it is not intended but is a foreseen side-effect
(2) When those who may be killed or let die have done something such that the "moral barriers" on not killing them have been lowered, or they have compromised their "inviolable status", or they have made themselves "liable to action" that may or will result in their death
e.g. killing or letting die human beings in self-defense, capital punishment, or in war
(3) Those who may be killed or let die are such that their metaphysical and moral status is uncertain and controversial -- they are "at the margins of life"
e.g. animals, human zygotes, embryos and fetuses, newborn infants, anencephalic infants, congenitally retarded human beings, human beings who have suffered severe brain damage or dementia, human beings who have become irreversibly comatose
(4) Those who may be killed or let die are such that death would be a benefit to them (they may have a desire to die, and they may have requested to be killed or to be let die)
e.g. euthanasia, suicide, assisted suicide
McMahan's first volume concentrates on circumstances of the third and fourth kind. (The second volume concentrates on circumstances of the second kind.)
The examination of the justification of killing and letting die of human beings and animals in circumstances of the third and fourth kind, in particular, has implications for the eating of meat, animal experimentation, infanticide, embryo research, use of anencephalic infants as organ donors, the termination of life support for the irreversibly comatose, the withdrawal of life-support for demented or incompetent patients in accordance with their earlier advance directives, and pre-natal injury.
In particular, however, it has implications for abortion. This is the main subject of McMahan's first volume. Since abortion is the killing or letting die of human zygotes, embryos and fetuses, and since at least part (if not all) of the justification for the killing or letting die of the same is that their metaphysical and moral status is uncertain and controversial, McMahan attempts to provide an account of their metaphysical and moral status, as well as that of animals, severely cognitively impaired human beings, etc., that will eliminate this uncertainty and hence end this controversy.