Given the criteria we have for considering whether a being is conscious, it is reasonable to conclude that vertebrates and a large number of invertebrates are conscious. The clearer cases are those of animals who have a centralized nervous system whose central organ (basically, a brain) has some development. However, there are a number of animals who possess centralized nervous systems whose central organ is not quite developed. In these cases doubts may arise about whether they are conscious or not. The reason for this is that if in order to be conscious it is necessary that a nervous system be organized in a certain way, then the evolutionary path that will lead there will necessarily pass first, in its previous stages, through the existence of a nervous system without any centralization, and afterwards through a nervous system that starts to be centralized, but not enough to host consciousness. First, the nervous system becomes minimally centralized, with some very simple nervous ganglia, then, with more complex ganglia. Nervous systems become more complex until at some point the phenomenon of consciousness appears. Along the evolutionary path there may be stages where there are some minimally centralized nervous systems that don’t give rise to consciousness.
We don’t know with full certainty if there are currently animals with minimally centralized nervous systems that don’t give rise to consciousness. It may be that all the centralized nervous systems that exist currently are centralized enough to host consciousness. This would be the case if all those that were in the intermediate stage, that is, having minimally centralized nervous systems that don’t give rise to consciousness, were already extinct. We have no answer to this question at this point.
Vertebrates and many invertebrates are conscious
Among those animals that are conscious we can count with a high degree of certainty vertebrates including human beings and invertebrates such as cephalopods (such as octopuses and squids), since they satisfy the criteria for sentience. In addition, we also have strong reasons to think that other animals such as arthropods (insects, arachnids and crustaceans) are conscious too. The physiology of these animals is organized in ways that seem to be sufficient for giving rise to consciousness, and their behavior also seems to support this idea.1
As for other animals, such as bivalve mollusks, we don’t have reasons as strong as those we have in the previous cases.2 However, given the problems involved in determining the basis of consciousness, we cannot rule out completely the possibility that they are sentient, unlike in the case of those with a nervous system that is not centralized.
The following are some examples of animals who would fall, respectively, in these two groups.
Insects and other arthropods
It is often a controversial issue whether animals such as insects, arachnids and other arthropods are sentient.3
In the case of insects we can consider the following line of reasoning, which is actually an argument by homology. Insects possess a centralized nervous system which is centralized not merely due to the presence of ganglia, but actually includes a brain. It must be noted, though, that it is a very simple and small brain. Therefore, considering insects’ physiology alone is not enough to conclude whether they are conscious or not. Apart from this, the behavior of some insects is very simple. Others, however, have very complex behavior. A clear example of this is bees. Their behavior, including their famous waggle dance, leads us to think that they really are beings with experiences, that is, they are conscious.4 There are other insects that have a very similar physiological structure to that of bees but that exhibit only much simpler behaviors, such as mosquitoes. Because of the similarity of their nervous systems, we might believe that if bees are conscious, then they are conscious, too. We must bear in mind, though, that this does not follow automatically. We must not lose sight of the fact that insects are the most numerous class of animals currently existing. Due to this, there are certain differences among them that are much more significant than those that can occur among mammals, for instance.
Because of this greater variation among insects, a different response may be to claim that bees (or, in general, hymenopterans, the insect order to which bees belong and which includes wasps and ants) are conscious, while other insects are not. Or, maybe, that even if all insects are conscious, bees are able to have more vivid experiences. This seems more likely to be the case than that only some insects are sentient. Although the differences in the behaviors of insects are very significant, the differences between their physiologies are not so important as to lead us to conclude that only some of them are sentient.
Of course, a different line of reasoning is also possible. We might think that beings exhibiting only simple behaviors could not be sentient. From here we could posit that the structure of the nervous systems of these animals would not be complex enough for consciousness to appear (despite its centralization). Therefore, we would conclude that, since their nervous systems are similar to those of animals exhibiting only simple behaviors, animals such as bees would not really be conscious, since they would lack the necessary nervous structure. We would then claim that even behaviors as complex as those of bees could occur through mechanisms that would not imply the presence of consciousness. This explanation, however, seems less plausible than the previous one, that complex behaviors imply consciousness and that, since some insects show complex behaviors, the nervous systems of all insects are similar enough that all insects must be conscious, though possibly to varying degrees. A being may be conscious and display a relatively simple behavior. It seems more unlikely, though, that a nonconscious being would display a complex behavior.5
In the same vein, we could consider other criteria as well, such as the presence of what are called natural opiates among insects. This would reinforce the claim that these animals are sentient.
In the case of other arthropods, such as arachnids for instance, we cannot appeal to evolutionary logic to apply the conclusions we reach in the case of insects, given that they are not closely related. Despite this, we may follow an argument from homology. Insects’ nervous structures are not significantly more complex than arachnids’. Plus, the behavior of arachnids is not very different from that of a number of insects. Therefore, it may make sense to infer that if insects are sentient, then arachnids are sentient too.
We can see that we are facing a question to which we cannot arrive at an immediate and clear answer. However, we can consider all the different criteria we have to examine the question together, and ponder all the evidence we have in order to make progress towards finding which is the most plausible answer. In fact, the reasoning process is similar to the one that is followed in the case of other animals (such as, for instance, vertebrates). It is only that here we may need to pay attention to more factors.
Bivalves and other beings that have centralized nervous systems with ganglia
The problem becomes more complex if we consider other beings with a simpler structure, that do not actually have a brain, as insects do, but only some central nervous ganglia, as, for instance, bivalve mollusks.6 This happens in the case of many of invertebrates. The appeal to evolutionary logic in these cases is not useful, since the behavior these animals display is very simple. It could be performed without requiring that the animals that display it be conscious. This happens in particular in the case of animals that stay attached to rocks or other surfaces without moving, in the case of bivalves or of certain crustacea such as barnacles. Bivalves can perform some movements, such as opening and closing their shells. But these movements may be triggered in a more economic way in terms of energy by some stimulus-response mechanism (in fact, their behavior is not more complex than that of other beings without a centralized nervous system, such as carnivorous plants or certain echinoderms). At any rate, their physiology leaves the question open.2 It might be that they have experiences. It is not possible to rule out that possibility given our lack of knowledge regarding how to answer the question of what is the basis of consciousness.
There are other indicators that are not conclusive, although they may help us to appraise the question. Bivalves possess mechanisms that are analogous to opiate receptors possessed by other animals.7 In other animals, the function of these receptors is to make it possible to have their suffering relieved when they are in significant pain. Due to this, a very plausible explanation of why bivalves have them, maybe the most plausible one, is that they can suffer too. But this is not totally conclusive. It is also possible that the organisms of these animals use these substances with an aim that is different from the one that they have in other animals.
Apart from these, there are other reasons that support the idea that bivalves and other animals with very simple centralized nervous systems can suffer. One of them is that some bivalves have simple eyes, and the most plausible explanation is that a being with eyes also has the experience of vision.8 In addition, it has been discovered that the heart rate of bivalves speeds up in those situations in which they are threatened by predators.9 These indicators, again, are not totally conclusive, but they show that it is not clear that these animals are not conscious. In the case of other animals that can have nervous systems with some centralization we can say something similar.
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5 This is a central question when it comes to how positive and negative experiences are spread in nature that is asked in a groundbreaking work in the examination of the suffering of animals in nature, Ng, Y.-K. (1995) “Towards welfare biology: Evolutionary economics of animal consciousness and suffering”, Biology and Philosophy, 10, pp. 255–285.
6 Bear in mind that others mollusks, such as cephalopods, have totally different nervous systems which are much more complex.
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9 Kamenos, N. A.; Calosi, P. & Moore, P. G. (2006) “Substratum-mediated heart rate responses of an invertebrate to predation threat”, Animal Behaviour, 71, pp. 809-813.