Sentience is the capacity to be affected positively or negatively. It is the capacity to have experiences. It is not the mere capacity to perceive stimuli or react to some action, as in the case of a machine that performs certain functions when we press a button. Sentience, or the ability to feel, is something different, namely the ability to receive and react to such stimuli consciously, by experiencing them from the inside.
Being sentient means being conscious
A conscious being is a subject of experience, meaning an entity that can experience what happens to itself. An organism can only be a subject of experience if organized in such a way that they have the capacity for consciousness, and if there are structures such as a nervous system that function to actually give rise to consciousness.
‘Being conscious’ is synonymous with ‘having experiences’. To say that someone has the experience of something is equivalent to saying that he or she is conscious of it. In other words, to be conscious is synonymous with being sentient (capable of having positive and negative experiences). Thus, when a creature is no longer conscious, it can no longer have experiences, and so ceases to be an individual, a subject. For example, when someone has an accident that irreversibly destroys the capacity for consciousness, the subject ceases to exist, even if the body is still alive.
To be sentient is to be capable of being affected positively or negatively
The reason that consciousness or sentience is crucial for morality is that experiences, which only conscious beings can have, can be positive or negative for individuals who possess them. They can affect the being for good or ill. A similar way of defining sentience, therefore, is the capacity to be harmed or benefited.
There are some important clarifications to make about the meanings of the terms ‘harm’ and ‘benefit’.
There are certain objects that can be damaged, but not harmed. There are objects with which we can perform certain functions, like a hammer, and objects that can perform certain functions for us on their own, like a car. If something happens to these objects so that they can no longer perform these functions, we say they are damaged. But that kind of damage is quite different from the ways in which a sentient being can be harmed. An object can’t be harmed. An object can’t be aware of the damage that has been done to it, or itself be affected by the damage in any way, because the object is not an individual who can feel suffering or enjoyment.
Use of the terms ‘suffering’ and ‘enjoyment’
Generally, positive experiences have been referred to with terms like ‘enjoyment’, ‘wellbeing’ or ‘happiness’, and negative experiences with terms like ‘pain’, ‘suffering’ and other synonyms. This terminology can sometimes be misleading, since it may lead one to believe that it refers only to certain kinds of positive or negative experiences. Specifically, the words ‘enjoyment’ and ‘suffering’ are often identified with physical pleasure and pain. And, at other times, they are equated with certain positive and negative experiences of a slightly broader character, including psychological suffering and pleasures. But they might not include things such as the sense of satisfaction someone feels after finishing an important piece of work.
All this can be confusing and misleading, however. If we use these terms to discuss sentience, it must be as synonyms for any kind of positive or negative experience, of any form of consciousness that feels good or bad to the individual. Assuming, according to this, that negative experiences can be broadly referred to by the term ‘suffering’ and positive experiences by the term ‘enjoyment’, another name for ‘sentience’ might be ‘capacity for suffering and enjoyment’. Those who prefer to use the terms ‘suffering’ and ‘enjoyment’ in a more restricted sense (such as to refer only to physical pains and pleasures) shouldn’t use the expression ‘capacity for suffering and enjoyment’ as synonymous with sentience. Authors who use this expression often use it to mean sentience, that is, to mean all kinds of positive and negative experiences.
Mental states are experiences
Another synonym sometimes used for ‘conscious’ or ‘sentient’ is ‘having mental states’. A mental state can only be experienced by a mind. A mind is simply a subject of experience. A state of mind is thus any kind of experience, even a very simple one such as feeling a physical pain or pleasure. Yet often the word ‘mind’ is used in a different way, to mean certain complex cognitive functions or certain complex intellectual abilities associated with thinking and learning. Understood in this sense, the term ‘mind’ means something very different from what the terms ‘sentience’ and ‘consciousness’ mean. Possessing certain complex intellectual abilities is not required for having what is technically called a mental state; all that is required is the possession of consciousness, however many other cognitive faculties may be lacking. Given this, there are strong reasons to think many nonhuman animals have mental states.
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