In animals, the brain is the control center of the central nervous system, responsible for behavior. In mammals, the brain is located in the head, protected by the skull and close to the primary sensory apparatus of vision, hearing, equilibrioception (balance), sense of taste, and olfaction (smell). While all vertebrates have a brain, most invertebrates have either a centralized brain or collections of individual ganglia. Some animals such as cnidarians and echinoderms do not have a centralized brain, and instead have a decentralized nervous system, while animals such as sponges lack both a brain and nervous system entirely. Brains can be extremely complex. For example, the human brain contains roughly 100 billion neurons, each linked to as many as 10,000 other neurons. The mind-body problem is one of the central problems in the history of philosophy. The brain is the physical and biological matter contained within the skull, responsible for electrochemical neuronal processes. The mind, in contrast, consists in mental attributes, such as beliefs, desires, perceptions, and so on. There are scientifically demonstrable correlations between mental events and neuronal events; the philosophical question is whether these phenomena are identical, at least partially distinct, or related in some other way. Philosophical positions on the mind-body problem fall into two main categories. The first category is dualism, according to which the mind exists independently of the brain. Dualist theories are further divided into substance dualism and property dualism. René Descartes is perhaps the most prominent substance dualist, while property dualism is more popular among contemporary dualists like David Chalmers. Dualism requires admitting non-physical substances or properties into ontology, which is in apparent conflict with the scientific world view. The second category is materialism, according to which mental phenomena are identical to neuronal phenomena. A third category of view, idealism, claims that only mental substances and phenomena exist. This view, most prominently held by 18th century Irish philosopher Bishop George Berkeley, has few contemporary adherents |