Intheory,wecouldalwaysbemistakenwhenweassumethat
otherhumanbeingsfeelpain.Itisconceivablethatoneofour
closefriendsisreallyacleverlyconstructedrobot,controlled
byabrilliantscientist soastogiveallthesignsoffeeling
pain,butreallynomoresensitivethananyothermachine.We
canneverknow,withabsolutecertainty,thatthisisnotthe
case.Butwhilethismightpresentapuzzleforphilosophers,
noneofushastheslightestrealdoubtthatourclosefriends
feelpainjustaswedo.Thisisaninference,butaperfectly
reasonable one, based onobservationsof their behaviorin
situationsinwhichwewouldfeelpain,andonthefactthat
we have every reason to assume that
ourfriendsarebeingslikeus,withnervoussystemslikeours
thatcanbeassumedto functionasoursdoand toproduce
similar feelings in similar circumstances.
Ifitisjustifiabletoassumethatotherhumanbeingsfeelpain
aswedo,isthereanyreasonwhyasimilarinferenceshould
be unjustifiable in the case of other animals?
Nearlyalltheexternalsignsthatleadustoinferpaininother
humanscanbeseeninotherspecies,especiallythespecies
mostcloselyrelatedtous—thespeciesofmammalsandbirds.
The behavioral signs include writhing, facial contortions,
moaning,yelpingorotherformsofcalling,attemptstoavoid
thesourceofpain,appearanceoffearattheprospectofits
repetition,andsoon.Inaddition,weknowthattheseanimals
have nervous systems very like ours, which respond
physiologically as ours do when the animal is in
circumstancesinwhichwewouldfeelpain:aninitialriseof
bloodpressure,dilatedpupils,perspiration,anincreasedpulse
rate,and,ifthestimuluscontinues,afallinbloodpressure.
Although human beings have a more developed cerebral