Which Beings Are Sentient?

Original text Luciano Carlos Cunha · Volume II, p. 16 of the PDF edition · English translation

4.1. Why is it important to know which beings are sentient?

It is important to know which beings are sentient because sentience is what makes a being capable of being harmed and benefited. This is so because it is sentience (that is, the capacity to have experiences) that makes that being someone rather than something, and because experiences can be experienced as positive or negative (that is, they have valence⁹).

There are three main types of criteria for investigating which beings are sentient: physiological, behavioral, and evolutionary¹⁰. We will look at each of them below.

4.2. The presence of a centralized nervous system

The main physiological criterion is the presence of a nervous system with a centralizing organ (such as a brain or ganglia, for example). Information reaches the organism through the nervous system, but it is the processing of that information, which takes place in the centralizing organ, that provides the conditions for consciousness to arise¹¹.

This means that organisms that lack a nervous system (such as bacteria, protozoa, fungi, plants, and animals of the phylum Porifera, such as sponges) or that have a nervous system but no centralizing organ (such as animals of the phylum Cnidaria, such as jellyfish, and of the phylum Echinodermata, such as sea cucumbers) lack the structures necessary for consciousness to arise¹².

4.3. Other physiological indicators

This first type of criterion (the presence of a centralized nervous system) is the main one, with the following criteria being additional indicators that a being is sentient. That being so, it is possible for an organism to be sentient without meeting these other additional criteria.

However, it does not seem possible for an organism to be sentient by meeting the additional criteria without having a physical structure that meets the first criterion, since in that case there would be no way for the organism to process the information received by the nervous system and create the conditions for consciousness to arise.

Other additional physiological criteria are the presence in the organism of substances that act as painkillers¹³ and the presence of nociceptors¹⁴ (which detect harmful or potentially harmful stimuli and send this kind of information to the brain).

4.4. Behavior

The second type of additional criterion is behavioral¹⁵. If a being has a centralized nervous system, the way it behaves can be an additional reason to think it is sentient.

For example, if an animal's behavior changes in response to stimuli under various kinds of conditions, it is probably avoiding stimuli that generate negative experiences and seeking stimuli that generate positive experiences.

That being so, to investigate whether the way a being behaves is additional evidence that it is sentient, what matters is not investigating how it behaves in specific situations, but how it behaves in general.

4.5. Evolutionary advantage

The third type of additional criterion is evolutionary advantage. The traits most likely to be passed on are those that are most efficient at transmitting genetic information. The parts of the body responsible for creating the conditions for consciousness to arise require a considerable investment of energy on the part of the organism. For example, in humans up to 20% of the energy consumed is spent keeping the brain active¹⁶. For this reason, the traits responsible for creating the conditions for consciousness to arise probably prevailed because they provided an advantage in terms of transmitting genetic information (since, if that were not the case, other traits that use much less energy would most likely have prevailed).

Thus, there is an evolutionary advantage for animals in being conscious, since, because they can move around in their environment, they are able to flee harmful stimuli and seek positive ones. This contributes indirectly to their survival, which in turn increases the likelihood that they will pass on their genetic information.

This is not the case, for example, with plants, since they cannot move. In their case, maintaining the structures necessary for consciousness to arise would involve a considerable expenditure of energy and would not result in any evolutionary advantage.

4.6. What is the minimum degree of centralization?

There is still no consensus on the minimum degree of centralization a nervous system must have (in terms of the number of neurons and the interaction among them) in order to provide the conditions for consciousness to arise.

It is known, however, that it need not be a brain like that of humans, nor like that of mammals, nor even like that of vertebrates¹⁷. For example, cephalopods meet all the criteria for sentience and are invertebrates¹⁸. There is also a series of studies suggesting that many kinds of insects, crustaceans, and other arthropods

are very probably sentient¹⁹. For this reason, it is a mistake to think that the sentience criterion excludes invertebrates from moral consideration.

4.7. The possibility of non-organic sentience

It is also possible that a being need not even be organic to be sentient. If sentience appears whenever a physical structure is organized in such a way that it performs certain functions, and this is independent of the material that makes up that physical structure, then it is possible that non-organic sentient beings²⁰ will come to exist in the future (for example, in digital media).

This may sound strange at first. However, there is a thought experiment that can help us understand this possibility. Imagine that we undergo surgery that involves replacing some of our organic neurons with neurons made of some non-organic material. If, after the surgery, the new neurons continue to perform the same functions, and the experiences associated with them remain present, this would be a reason to think that sentience is possible in non-organic beings. For example, if we kept periodically replacing the rest of our current neurons with non-organic neurons until all of them had been replaced, and the experiences did not fade away, this would be a strong indication that non-organic sentience is possible.

So far, every sentient being belongs to the animal kingdom. But this may change in the future.