The problem of consciousness To be conscious is to be able to have some kind of subjective experience or awareness of something.1 We can only experience something if we are conscious, and if we are conscious it means we can have experiences. Conscious creatures can experience something external in the environment or something internal to the body. It can be the experience of a feeling or of a thought of any type. An experience is positive when the subject enjoys it, is satisfied with it, or is pleased by it. It is negative when it involves some form of suffering. To suffer is to have a negative experience. All emotions and feelings that we have are experiences, and we can also have experiences that are caused only by our thoughts. We can have these experiences insofar as we are conscious; indeed, the fact itself of having experiences is, as noted above, to be conscious. The word “sentience” is sometimes used instead of consciousness. Sentience refers to the ability to have positive and negative experiences caused by external affectations to our body or to sensations within our body. The difference in meaning between sentience and consciousness is slight. All sentient beings are conscious beings. Though a conscious being may not be sentient if, through some damage, she has become unable to receive any sensation of her body or of the external world and can only have experiences of her own thoughts. When a creature has an experience, there exists in that creature what we can call a subject, that is, a “someone” who is having the experience, an “I” who is conscious. The word subjective, which refers to inner, or personal, experiences, refers to this subject. A subject is a someone, one who experiences their world, as an animal does. An object is a thing that does not experience its world. A chicken is a subject of experience, whereas a rock is not. If you pet a chicken she will feel pleasure. If you pet a rock, there is no one there to feel anything. The question we have to answer is: what sorts of creatures are sentient (and, therefore, conscious)? Or, put another way, what kind of physical structure and arrangement of nerve cells does a creature have to have so that it isn’t merely a collection of cells, but a conscious being?2 What is the problem of consciousness? The problem of consciousness can be formulated as follows: how is it that, from a purely material basis (a brain or a centralized nervous system), consciousness emerges?3 This is what the problem of consciousness really boils down to. Answering this requires answering the question, what structures must be present in an organism and how would they function for consciousness to be possible? In other words, of all the different ways that the bodies of animals are arranged, which ones contain structures and arrangements that give rise to consciousness? There is no reason to suppose that only a human-like central nervous system will give rise to consciousness, and a great deal of evidence that very different types of animals are conscious. An example is bird brains, which have many structural similarities to mammalian brains, but different arrangements of neurons. Yet their brain circuits seem to be wired in a different way that creates a similar effect in terms of consciousness and cognition. An octopus is an invertebrate with a very different type of nervous system. But an octopus exhibits behavior and responds to her environment like a conscious creature. Why is it that only beings with a centralized nervous system are sentient? We don’t yet know what causes consciousness to arise. And until we know this, we can’t know which creatures will be sentient. But we do know that, in the absence of at least a centralized nervous system, consciousness will not arise in an animal. By this we must understand a nervous system that not only transmits information, but has also some brain or ganglia that processes it. We know that creatures lacking a centralized nervous system cannot be conscious. Non-centralized nervous systems do transmit information about damage in some part of the organism, but this information does not result in a conscious experience because there is no bodily structure in which a sufficiently large aggregate of nerve cells interact to process an experience, as opposed to merely transmitting the information. It is the processing of information that produces the experience. Processing or computing information is not merely an indication of consciousness. Consciousness seems to be impossible if no processing occurs. Reflex arcs: How a nervous system operates without giving rise to an experience In our bodies, if our knee is lightly tapped, our leg moves automatically (with no intention on our part) and independently of the experience of the tap that we sense. The information that originates in our knee, with the tap, splits up and moves through two separate pathways: one path goes to our brain through the spinal cord, where it is processed to produce the corresponding experience; the other path involves a different circuit, going through the spinal cord to the muscles that operate the leg, without ever reaching the brain. In the second path the information takes a much shorter direct route to enable our body to react quickly to the stimulus (‘reflex arc’). There is a good reason why this dual mechanism exists. There are cases where some part of the body will be endangered by a slow reaction to an external threat. If we had to think about moving because of pain, rather than responding automatically, we might not act quickly enough to avoid harm. What is relevant here is that the information transmitted through this ‘reflex arc’ is never experienced because it is never processed by a central nervous system. The non-centralized nervous systems of some animals operate just as reflex arcs do. Information is transmitted from the cells receiving certain stimuli to other cells which must be activated, without any involvement of subjective experience. In these cases there is a merely mechanical transmission of information. Such reactions are not an indication of sentience. For this reason, we can rule out the hypothesis that beings without a centralized nervous system are sentient, just as we can for organisms lacking a nervous system altogether (see Which beings are not conscious). What is known about the emergence of consciousness? How do the structures and arrangements of different centralized nervous systems operate to give rise to consciousness? We don’t know. Currently researchers are trying to identify the neural correlates of consciousness in humans. The neural correlates of consciousness are the “neural events”, i.e., the ways that sets of neurons work and operate when a given mental operation occurs.4 In connection with this, researchers are studying human subjects who have suffered brain lesions and who have, as a consequence, lost some aspects of consciousness. These studies are in their infancy and it will take a very long time before we have a solid understanding of the neural correlates of consciousness. Knowing what operations take place in a nervous system when some experience occurs does not explain how those operations create the experience. And the neural correlates of a certain type of experience might be different in different types of animals, like birds, cetaceans and cephalopods. We just don’t know. Such early research can provide only limited knowledge and while the problem of what consciousness is and how it arises remains unsolved, speculations about how centralized nervous systems produce experiences will remain open to revision. Due to the difficulty of solving the problem of consciousness, those who study it agree that it is unlikely that it will be solved in the near future. Given what we know today, we can only make rough estimates about which beings are more or less likely to be sentient and we can confidently assert that certain creatures are not sentient. Given current information, it is impossible to know with certainty which beings with centralized nervous systems are conscious. We know that without such a system there cannot be consciousness, but we do not know what degree of complexity such a system must possess for consciousness to emerge. We cannot know exactly which beings can have experiences until we know exactly what physical basis is necessary for consciousness and therefore experiences. And we cannot answer this question until we solve the problem of how consciousness arises. The significance of having experiences that are positive or negative In order to determine which beings to give moral consideration to, we must consider that beings who have experiences as a result of the evolutionary process can have both positive and negative experiences.5 If there were beings who had either positive or negative experiences only, these beings would also deserve moral consideration. There could also be entities that have experiences that are neither positive nor negative. There is a difference between the capacity to have experiences in general and the capacity to have positive or negative experiences specifically. It may be possible to create a computer that can have experiences yet is indifferent to those experiences. Its experiences would be neither positive nor negative. The computer wouldn’t care whether it has them or not. Such a computer would also be indifferent towards its own continued existence. Because it would lack positive and negative experiences altogether, the computer wouldn’t care how we treated it. Regardless of what we did to the computer, it would be impossible for us to harm or help it.