The definition of sentience is 'the ability to perceive or feel things'. Sentient creatures have the ability to receive information from their environment and to feel and react to that information, whether they show fear, happiness or indifference.
Your dog might display his or her sentience when they react to a doorbell ringing by barking. Your dog is noisily trying to alert you to a change in your environment by barking at you. The same creature might react with happiness when you return home from a day at work. Your dog has missed you and is expressing his or her joy at your return.
Your houseplant, on the other hand, is not sentient and will not react to changes in environment in the same way. Plants react to stimuli, but this is not the same as being sentient. To be sentient, a creature must have a central nervous system and the capacity to perceive an environment and think through reactions.
Are animals sentient creatures?
Yes, animals are sentient. Scientists do not dispute animal sentience. It is an absolute fact that animals have sentience. They are capable of drawing from their environment, perceiving situations and feeling. Animals are as sentient as you or I.
Lots of people try to dispute the sentience of animals because it makes them feel better about eating animals for food. If animals were not sentient, the process of slaughtering them would be much less traumatic and painful for chickens, pigs, sheep, cows and other sentient beings. In reality, we cannot justify the slaughter of animals on the basis that they feel any less than we do, because animals have been scientifically proven to be conscious, sentient beings.
Are plants sentient?
No, plants are not sentient. They are most certainly alive, but they are not sentient. Plants do not develop interests or tastes, preferences or opinions in the way humans and sentient non-humans do. Plants do not have nervous systems in the same way sentient beings are. Without these, they cannot be sentient.
Your central nervous system comprises of your brain and spinal cord. It is responsible for the control of your thought processes, movement, sensations, thoughts, speech and memory. Plants do not have a central nervous system, which means they don't store memories, experience things, or had a sense-of-self or consciousness.
Evolutionarily, this makes perfect sense. There would be no point in plants developing sentience when they cannot do anything to react to a change in their environment. This would do nothing but cause them pain and distress. The absence of sentience in plants protects them from torture and trauma.
Vegans and sentience
If you are vegan, you have no doubt been challenged to defend eating plants on the basis that they are killed for us to eat them. Our first defence when asked is usually to declare that animals are sentient and plants are not. This is true, but it is important that you try to explain this further if you can in order to help people to understand.
Animals have a central nervous system, which means they react to things in the same way humans do. They can feel excitement, joy, happiness, contentment and, on the other side of that, stress, fear, anxiety and terror. If someone causes them physical pain, they are conscious of that. It causes them great fear and stress in the same way pain causes a physical and emotional response in humans.
Plants do not have this capacity. They can react to a stimulus by moving towards light or away from extreme heat, for example, but they do not have the sentience required to think these actions through in the way that a human or a non-human animal can. If you were to make a loud, terrifying noise in your house, your dog or cat, as a sentient being, might run away and hide. At the very least, he or she would throw you a questioning glance. Your house plant would not, because it is not sentient. (Please do not make terrifying noises at your animal friends).
If that fails, ask your challenger if most humans would have the same reaction to watching the icking and chopping up of as apple as they would the slaughter and chopping up of a chicken. They would not. The reason they would not is due to sentience. We are aware of the terror and pain suffered by sentient beings and it causes us distress. It is the reason animals in slaughterhouses cry and react with fear when they see other animals killed. Plants, on the other hand, do not struggle or try to run away because they do not have sentience, and because of that they can't react to their environment, so are not distressed when they see the suffering of another creature.
What is sentience in animals?
Sentience gives animals the capacity to feel enjoyment, happiness, joy, excitement, fear, terror, anxiety and panic. It gives humans and non-human animals the ability we have to react to our environment, by running away, for example, if someone threatens us or names us feel uncomfortable.
Sentientism is the reason your cat gets excited when you rattle a food bag or your dog expresses their excitement when you return home. It's the reason pigs scream in fear when a slaughterman approaches them with a knife or cows throw themselves from torture trucks. Animals have the same abilities we do when it comes to reacting to their environments and the things they have learned from their experiences.
Sentience and ethics
As conscious beings, humans have an ethical responsibility to respect the sentience of our fellow Earthlings. We know how it feels to be afraid or alarmed. We can comprehend pain, because most of us have felt it in some capacity at some point in our lives.
Vegans respect the sentience of other creatures and therefore seek to exclude their use from our lives. We believe that to subject animals who feel, think, love and experience in the same way we do to slaughter is cruel and unnecessary. This is an ethical decision we have made.
You can live a healthy life without consuming the bodies of animals and subjecting sentient being to pain and fear. by switching to a plant-based diet and excluding products like leather and silk from your lifestyle. There are lots of vegan alternatives for clothes and tonnes of accidentally vegan foods. You might be surprised to find that a lot of the foods you're eating now are already vegan.
Sentience and fish
Fish are generally forgotten in arguments of sentience. There are sub-standard rules around the slaughter of cows, pigs, chickens and other farm animals on the basis of their sentience, but the same regards are rarely given for fish. For some reason, they are excluded from the moral circle. Perhaps it is because they cannot scream or cry out in pain. Still, science has confirmed that fish feel pain.
Fish are sentient and emotional beings. They feel pain and fear. Still, our current slaughter laws allow fisherman to remove them from the water and let them die on the deck of a ship. When fish are pulled from the water, they feel fear and pain. They are fighting for their lives in the way any other sentient creature would.
When humans drown, it takes four or five minutes. For fish, this process is much longer. Their deaths are prolonged, painful and full of fear.