Animal sentience has received little attention by those who have studied the characteristics and capabilities of nonhuman animals. This is unfortunate, because it is a very important issue. Sentience is what matters when we consider which beings to give moral consideration to. Due to this, we need to study the issues of what physical structures are necessary for sentience, which beings are sentient, and what interests sentient beings may have.
In order to understand these problems there are several questions that must be addressed.
Being sentient is crucial when it comes to respecting someone. What does it mean, then, to be sentient?
In order to be sentient an animal needs to have a physical structure such as a centralized nervous system that can host consciousness. But a lot remains to be learned about this.
How can we guess whether a certain being is sentient or not? There are several criteria that nonhuman animals satisfy that give us reasons to conclude they are.
Those who deny that nonhuman animals are sentient sometimes claim that it cannot be demonstrated whether animals are sentient. But there are powerful reasons to reject such a view.
The field of animal cognition has been the subject of much interest, much more than the question of which animals are sentient. Yet animal cognition is a subject of less importance, since having any kind of intellectual abilities is not a necessary condition for having the capacity to feel (sentience).
Sentience, or the ability to have experiences, is sometimes mistaken for the ability to experience the existence of oneself. These faculties are different: technically one can be conscious without being self-conscious (although it is a subject of debate whether consciousness is really possible without self-consciousness).
Given the criteria we have to consider whether a being is conscious, there are reasons to conclude that animals who are sentient and therefore conscious have centralized nervous systems.
There are animals that don’t have the physical structures that are needed in order to have experiences. They include those that lack any nervous system and those whose nervous systems are not centralized.
Even if we conclude that many nonhuman animals are sentient beings, we may still not be able to recognize situations in which they are suffering. There are several indicators that can help us determine when animals are suffering.