4 0 3W e i g h i n g fa c t o r swell-being. Nevertheless, sentient beings may be said to have a basic inter-
est in pleasure and the avoidance of pain. Respect for this basic interest is
central to utilitarian ethics.
The sentience criterion suggests that, other things being equal, it is morally
worse to kill a sentient than a non-sentient organism. The death of a sentient
being, even when painless, deprives it of whatever pleasurable experiences it
might have enjoyed in the future. Thus, death is apt to be a misfortune for
that being, in a way that the death of a non-sentient organism is not.
But how can we know which living organisms are sentient? For that mat-
ter, how can we know that non-living things, such as rocks and rivers, are
not sentient? If knowledge requires the absolute impossibility of error, then
we probably cannot know this. But what we do know strongly suggests that
sentience requires a functioning central nervous system — which is absent
in rocks, plants, and simple micro-organisms. It is also absent in the early
human fetus. Many neuro-physiologists believe that normal human fetuses
begin to have some rudimentary capacity for sentience at some stage in the
second trimester of pregnancy. Prior to that stage, their brains and sensory
organs are too undeveloped to permit the occurrence of sensations. The
behavioural evidence points in the same direction. By the end of the first
trimester, a fetus may have some unconscious reflexes, but it does not yet
respond to its environment in a way suggestive of sentience. By the third
trimester, however, some parts of the fetal brain are functional, and the fetus
may respond to noise, light, pressure, motion and other sensory stimuli.
The sentience criterion lends support to the common belief that late abor-
tion is more difficult to justify than early abortion. Unlike the presentient fetus,
a third-trimester fetus is already a being — already, that is, a centre of experi-
ence. If killed, it may experience pain. Moreover, its death (like that of any
sentient being) will mean the termination of a stream of experiences, some of
which may have been pleasurable. Indeed, the use of this criterion suggests
that early abortion poses no very serious moral issue, at least with respect to
the impact upon the fetus. As a living but non-sentient organism, the first-
trimester fetus is not yet a being with an interest in continued life. Like the
unfertilized ovum, it may have the potential to become a sentient being. But
this means only that it has the potential to become a being with an interest in
continued life, not that it already has such an interest.
While the sentience criterion implies that late abortion is more difficult to
justify than early abortion, it does not imply that late abortion is as difficult
to justify as homicide. The principle of respect for the interests of sentient be-
ings does not imply that all sentient beings have an equal right to life. To see
why this is so, we need to give further thought to the scope of that principle.
Most normal mature vertebrate animals (mammals, birds, reptiles, am-
phibians and fish) are obviously sentient. It is also quite likely that many
invertebrate animals, such as arthropods (e.g. insects, spiders, and crabs),
are sentient. For they too have sense organs and nervous systems, and
often behave as if they could see, hear, and feel quite well. If sentience is97364_ch19_ptg01_383-422.indd 403 11/15/13 5:45 PM
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