There are three general criteria for deciding whether a being is sentient. These involve considerations that are behavioral, evolutionary and physiological.
Behavior
When we experience suffering or enjoyment, we tend to behave in
certain ways. We grimace, we cry, we groan… And the same is true of
other sentient beings. This applies to both human beings and a large
number of nonhuman animals. Behavior of this sort indicates that
those who behave in these ways are having positive or negative
experiences.1
There are, furthermore, certain types of behavior that may lead us
to suppose that a creature might be having such experiences, namely
those that demonstrate an understanding of beneficial or harmful
aspects of the environment. For instance, we may see that an animal,
after being burned for the first time, will stay away from fire in
the future. And the same applies to positive experiences, as when an
animal finds food at a certain location and later returns to that
spot. However, this behavior alone doesn’t provide a reason to
believe that these creatures can experience suffering and enjoyment.
It is, more generally, a reason to believe that they can have
experiences at all and are therefore conscious. Although we should
also note that it is perfectly possible that there are beings who
are conscious but lack any capacity for learning.
These are examples of specific behaviors exhibited by many nonhuman
animals. But these creatures behave in complex ways not only in
situations where we may think that they are experiencing suffering
or enjoyment. What is most relevant to ascertain whether a being is
sentient is not how that being reacts in these specific cases, but
how the being behaves in general. The behavior of an animal can lead
us to understand that she is sentient, even if she doesn’t exhibit
signs of suffering or enjoyment. Here’s the reason.
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