Beings that have no centralized nervous systems are not sentient. This includes bacteria, archaea, protists, fungi, plants and certain animals. There is the possibility that a number of animals with very simple centralized nervous systems are not sentient either, but this is an open question and cannot be settled yet.
The reasons that lead to this conclusion are as follows:
Only among animals can we find the physical structures that enable sentience
Possession of a centralized nervous system is what enables animals to have experiences, and only animals possess such systems. No other living entity has a nervous system. Looking at the anatomy of a fungus, bacteria or plant, for example, we will not find any nerves.
It could be that beings other than animals possess different physical structures that fulfill the same function as a centralized nervous system. Thus, a system organized in an equally complex fashion could result in a sentient organism. This is, in principle, entirely possible. However, among all organisms in our biosphere, none of the non-animals such as plants, fungi, protists, bacteria and archaea has such a structure. None of them has a mechanism for transmission of information similar to that present in animals with centralized nervous systems.
Evolutionary logic and living beings that aren’t animals
Structures that allow for the development of consciousness appear very early in the development of animals, yet do not ever appear in living things that are not animals. Living entities that are not animals have a very simple structure. They do not have a nerve structure or any physical structure complex enough to allow for the possession of consciousness. Moreover, the possession of such a structure would make no evolutionarily sense.
As shown in What beings are conscious, the capacity to feel arises in evolutionary history due to its usefulness in motivating animals, through positive and negative stimuli, to engage in or abstain from fitness-increasing behavior. Therefore, it would make no sense for beings who lack the capacity to engage in such behavior to have the capacity to feel. For example, plants can’t run away from a threat or forage for a type of food they enjoy. These stimuli would serve no purpose, and would involve an unnecessary expenditure of energy.
Plants do not have experiences: the response to external stimuli is not sentience
One idea that has no scientific backing but which has received some support is the view that plants have experiences because they respond to certain stimuli. However, exhibiting a physical response of this type does not require the capacity for subjective experience.
It is also sometimes claimed that certain plants grow better if there is music in the environment, or if people talk to them. It may be that certain sound waves somehow benefit plant growth, and that these waves overlap with those that humans find pleasant. But this by no means implies that plants are organisms with physical structures that cause mental experiences, a center of consciousness that allows them to experience and appreciate music and improve their growth on that basis (we may note that the taste in music is something very culturally specific, which further shows the absurdity of the pseudo-scientific assertion that “plants like music”). In any case, any other alleged evidence of this sort cannot be considered a sign of possession of consciousness by the plant as long as it is only based on behavioral observations. Arguments for possession of consciousness must be backed by physiological evidence, with a specific physical structure identified and reasons given for why such a structure might give rise to conscious experience.
The ways an organism lacking a non-centralized nervous system may respond to stimuli can vary greatly. Still, however complex they are, with no centralized nervous system or physical structure that can fulfill a similar function, such a response cannot be explained by consciousness. We should explain it by assuming some alternative physical mechanism. Although non-conscious physical responses fail to attain a level of complexity comparable to that of creatures whose consciousness allows them a wide range of behaviors, non-conscious responses can have a relatively high level of complexity.
This can also be seen in a number of machines that humans have manufactured. For example, a bulb connected to a photoelectric cell can be switched on and off depending on the amount of light in the environment, without this being accompanied by any type of experience.
The fact that only animals are sentient does not mean that all animals are sentient. As explained in the page on criteria for sentience, in order to have experiences it is necessary to have a centralized nervous system. And some animals lack such a system. This implies that there are animals who cannot be sentient. First, we would include here those beings that do not have a nervous system, such as Porifera (the phylum that includes sponges), and those who do have a nervous system which is not centralized, such as echinoderms and cnidarians. Non-sentient animals would then include sponges, corals, anemones, and hydras.
Again, as in the case of plants, these animals may react to external stimuli, and even engage in locomotion. For example, sponges, though not having a nervous system, have a physical mechanism that allows them to perform certain movements (by circulating water through the cells of which they are composed). Echinoderms (such as starfish, sea urchins and sea cucumbers) can have relatively complex behavior (as can, for example, a carnivorous plant). But, as in the case of plants, there is nothing in their physiology to allow possession of sentience.
Depending on what kind of organization a centralized nervous system needs in order to allow experience, it is possible that some animals with centralized but very simple nervous systems are not sentient. This could happen if consciousness requires a certain degree of nervous complexity, which may well be the case. However, since at present we lack the relevant knowledge, the question must remain open. What we do know based on our present knowledge is that all sentient beings are animals, but not all animals are sentient.
It is important to note, though, that there are many other animals who do have simple yet centralized nervous systems. This includes many invertebrates, including mollusks such as cephalopods and arthropods such as crustacea or insects. Our degree of certainty about whether they are sentient can vary (we can be really confident that they are in the case of cephalopods, but uncertain in the case of bivalves). But the case of these animals is wholly different from that of the animals without any nervous system with a structure that allows information processing.
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