Sentience

Sentience is the ability to feel, perceive and experience subjectively. Subjective experiences are often referred to as 'qualia'.

Sentience allows for the awareness of one's own surroundings, and the subjective interpretation of such.

What is sentience?

For a better understanding of what sentience is and what it implies, an understanding of where it originates and what process is required for its formation is necessary.

To see how sentience works, the formation of a subjective experience can be explained by showing the four steps required in the nervous system for it to exist:

1- The somatosensory system - which is responsible for being able to perceive things (perception of sight, noise, touch, taste, smell, pressure, pain, temperature, position, movement, vibration).

2- The thalamus - which is responsible for 'gathering' the sensory signals given by the somatosensory system.

3- The thalamocortical radiations - which are fibers between the thalamus and the cortex, and are the carriers of information between the thalamus (gatherer) and the cortex (interpreter), relaying the information from the former to the latter and allowing them to communicate, to be able to have the data interpreted subjectively.

4- The cortex, which is responsible for awareness and subjective interpretation of the information given. The mid-brain (the more primitive part) is also sufficient enough to allow for basic capacities for subjective experiences. [1]

To understand this more clearly, a comparison can be made with delivering a letter, where the letter is an information that can be interpreted subjectively:

1- Without the somatosensory system, there is no letter. The delivery person (thalamus) carries nothing, and the receiver (your awareness) receives nothing.

2- Without the thalamus, there is no delivery person. The letter is there, and you're ready to receive it, but nothing happens.

3- Without the thalamocortical radiations, you can't grab the letter from the delivery person and open it, nor can you and the delivery person see each-other. The letter exists, and it's ready to be delivered, but it's not able to be grabbed and opened.

4- Without the cortex, you don't know how to read the letter. The letter exists, it's delivered, and it's opened, but it's unreadable.

In all the cases, you won't know what the letter has to say (can't be aware/sentient).

Noting all these parts, it becomes apparent that sentience requires complexity and size.

That's why with microorganisms, for example, it's safe to assume they aren't sentient, simply because the small size doesn't allow for a complex enough neural network for the microorganisms to have sentience - they don't have the structure for it (being able to do movements and reacting to stimuli isn't sentience without all the other components). That's why certain insects like termites are in a grey area - they are probably a good example of what is at the edge of sentience vs non-sentience.

The more complex and 'evolved' these parts are, the higher degrees of sentience are there.

There's a direct correlation between volume/complexity of the nervous system and brain, and higher degrees of sentience.

The complexity and magnitude of the central nervous system (brain) and of the cognitive capabilities to process subjectively and interpret the information through the thalamocortical complex (which is the collective set of the thalamus, the thalamocortical radiations, and the cortex) are directly correlated with higher intelligence and more complex ability to experience subjectively in species.

This would make sense because of two things:

• A more complex nervous system and brain leads to more complex, and therefore more subjective and unique, experiences
• A higher amount of nerve endings and neurons leads to more intense feelings, perceived at a higher magnitude

Because a bigger structure for higher complexity is necessary, the bigger the brain, and the higher the cognitive capabilities, the bigger the animal usually is. As you can see in the image on the right, there's a correlation between animal size and amount of brain mass.

Considering how sentience has different levels of complexity, as there are beings of lower/higher levels of sentience, it'd be correct to see sentience as a spectrum, with fleas, bedbugs, termites and other very small insects on the lowest end, and elephants, whales, chimpanzees and humans on the highest end.

Why does sentience matter?

Once an understanding of what sentience is and of how it works is there, it's self-intuitive why sentience would matter morally.

The term morality is commonly used as the framework used to determine whether something is right or wrong, and the distinction between right and wrong.

To understand why sentience is at the core of morality, one must ask what morality would be without sentience. To determine what's right or wrong, consequences of actions have to be looked at, and the impact that they have - and to understand if something is right or wrong, the amount of suffering caused/going against someone's interests (something a sentient being would want to avoid) vs the amount of happiness/satisfaction brought (something a sentient being would want to pursue) has to be looked at.

Sentience is the line to determine whether something is aware and perceives, and has desires and wants, and sentience is the quality without which something can't be aware of anything, nor experience subjectively.

Therefore, without sentience, the consequences of your actions would have no impact whatsoever, as they couldn't be perceived and felt. Without sentience, there are no wants your actions could go against, nor beings that could feel pain as a result of your actions.

It then becomes clear that to have something that would matter morally (i.e. wants, feelings, subjective perception), sentience is an absolute requirement, and that morality wouldn't matter and wouldn't have meaning without sentience.

For example, in a universe without any form of sentient life forms, there would be no need to care for anything. No amount of destruction, or even complete annihilation, could inflict any amount of suffering - as there are no sentient life forms that can experience suffering.

Once sentience is brought to the equation, morality has meaning. To determine the right or wrong thing, one must take into consideration the potential suffering caused, and therefore the sentient life forms involved - as they are the ones and the only ones that would experience.

Sentient or non-sentient?

Not all animals are sentient, as explained above. Below are some examples of animals that aren't sentient, and some examples of animals commonly thought to be non-sentient that actually are.

Sponges

Sponges are certainly non-sentient.

They have 0 neurons and no nervous system. They cannot feel or perceive, nor can they posses any thoughts or experience. While biologically animals, they are effectively equivalent to a plant or a rock in terms of sentience.

However, moral consideration when dealing with sponges must be had in that they filter water, making them beneficial for others that are sentient.

Bivalves

Bivalves enter a more grey territory, even though they're most likely non-sentient.

While they do have nervous systems, they lack a central nervous system (brain) to process information.

They do react to stimuli, but, as made present before, a reaction to stimuli does not mean sentience, the same way a computer responding to a command or a sunflower following the sun is not equivalent to sentience.

Just like with sponges, bivalves filter and clean water, making them useful for sentient beings.

Another consideration to make is the risk that eating bivalves carries, considering that environmental contaminants and heavy metals are accumulated in their tissues because of the filtering they do.

Bivalves has been a strong controversy in the vegan community, with a side arguing for their chance to be sentient as a reason not to eat them, and the other side arguing for their impossibility of sentience as a reason to not exclude them. That said, rather than focusing on their questionable sentience--safe to assume to be non-existent--, bivalves are preferably avoided as a food regardless, because of the risks associated with eating meat and because of the contaminants they contain.