The mind is the set of faculties responsible for all mental phenomena. Often the term is also identified with the phenomena themselves. These faculties include thought, imagination, memory, will, and sensation. They are responsible for various mental phenomena, like perception, pain experience, belief, desire, intention, and emotion. Various overlapping classifications of mental phenomena have been proposed. Important distinctions group them according to whether they are sensory, propositional, intentional, conscious, or occurrent. Minds were traditionally understood as substances but it is more common in the contemporary perspective to conceive them as properties or capacities possessed by humans and higher animals. Various competing definitions of the exact nature of the mind or mentality have been proposed. Epistemic definitions focus on the privileged epistemic access the subject has to these states. Consciousness-based approaches give primacy to the conscious mind and allow unconscious mental phenomena as part of the mind only to the extent that they stand in the right relation to the conscious mind. According to intentionality-based approaches, the power to refer to objects and to represent the world is the mark of the mental. For behaviorism, whether an entity has a mind only depends on how it behaves in response to external stimuli while functionalism defines mental states in terms of the causal roles they play. Central questions for the study of mind, like whether other entities besides humans have minds or how the relation between body and mind is to be conceived, are strongly influenced by the choice of one's definition.

Mind or mentality is usually contrasted with body, matter or physicality. The issue of the nature of this contrast and specifically the relation between mind and brain is called the mind-body problem. Traditional viewpoints included dualism and idealism, which consider the mind to be non-physical. Modern views often center around physicalism and functionalism, which hold that the mind is roughly identical with the brain or reducible to physical phenomena such as neuronal activity though dualism and idealism continue to have many supporters. Another question concerns which types of beings are capable of having minds. For example, whether mind is exclusive to humans, possessed also by some or all animals, by all living things, whether it is a strictly definable characteristic at all, or whether mind can also be a property of some types of human-made machines. Different cultural and religious traditions often use different concepts of mind, resulting in different answers to these questions. Some see mind as a property exclusive to humans whereas others ascribe properties of mind to non-living entities (e.g. panpsychism and animism), to animals and to deities. Some of the earliest recorded speculations linked mind (sometimes described as identical with soul or spirit) to theories concerning both life after death, and cosmological and natural order, for example in the doctrines of Zoroaster, the Buddha, Plato, Aristotle, and other ancient Greek, Indian and, later, Islamic and medieval European philosophers.

Psychologists such as Freud and James, and computer scientists such as Turing developed influential theories about the nature of the mind. The possibility of nonbiological minds is explored in the field of artificial intelligence, which works closely in relation with cybernetics and information theory to understand the ways in which information processing by nonbiological machines is comparable or different to mental phenomena in the human mind. The mind is also sometimes portrayed as the stream of consciousness where sense impressions and mental phenomena are constantly changing.