Health
Health is the ability of a biological system to acquire, convert, allocate, distribute, and utilize energy with maximum efficiency. The World Health Organization (WHO) defined human health in a broader sense in its 1948 constitution as "a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity." This definition has been subject to controversy, in particular as lacking operational value, the ambiguity in developing cohesive health strategies and because of the problem created by use of the word "complete", which makes it practically impossible to achieve. Other definitions have been proposed, among which a recent definition that correlates health and personal satisfaction.
Mental Health
Mental health is a level of psychological well-being, or an absence of mental illness. It is the "psychological state of someone who is functioning at a satisfactory level of emotional and behavioural adjustment". From the perspective of positive psychology or holism, mental health may include an individual's ability to enjoy life, and create a balance between life activities and efforts to achieve psychological resilience. According to the World Health Organization (WHO), mental health includes "subjective well-being, perceived self-efficacy, autonomy, competence, inter-generational dependence, and self-actualization of one's intellectual and emotional potential, among others." The WHO further states that the well-being of an individual is encompassed in the realization of their abilities, coping with normal stresses of life, productive work and contribution to their community. Cultural differences, subjective assessments, and competing professional theories all affect how "mental health" is defined.
Neuroscience
Neuroscience (or neurobiology) is the scientific study of the nervous system. It is a multidisciplinary branch of biology, that combines physiology, anatomy, molecular biology, developmental biology, cytology, mathematical modeling and psychology to understand the fundamental and emergent properties of neurons and neural circuits. The understanding of the biological basis of learning, memory, behavior, perception and consciousness has been defined as "the ultimate challenge of the biological sciences".
Psychology
Psychology is the science of behavior and mind, including conscious and unconscious phenomena, as well as feeling and thought. It is an academic discipline of immense scope and diverse interests that, when taken together, seek an understanding of the emergent properties of brains, and all the variety of epiphenomena they manifest. As a social science it aims to understand individuals and groups by establishing general principles and researching specific cases.
Neuroscience
The elegant study... is consistent with the themes of modern cognitive neuroscience. Every aspect of thought and emotion is rooted in brain structure and function, including many psychological disorders and, presumably, genius. The study confirms that the brain is a modular system comprising multiple intelligences, mostly nonverbal.
Steven Pinker "On Einstein's brain," The New York Times (June 24, 1999)
Psychology
The Savage interrupted him. "But isn't it natural to feel there's a God?"
"You might as well ask if it's natural to do up one's trousers with zippers," said the Controller sarcastically. "You remind me of another of those old fellows called Bradley. He defined philosophy as the finding of bad reason for what one believes by instinct. As if one believed anything by instinct! One believes things because one has been conditioned to believe them. Finding bad reasons for what one believes for other bad reasons–that's philosophy. People believe in God because they've been conditioned to.
"But all the same," insisted the Savage, "it is natural to believe in God when you're alone–quite alone, in the night, thinking about death …"
"But people never are alone now," said Mustapha Mond. "We make them hate solitude; and we arrange their lives so that it's almost impossible for them ever to have it."
Aldous Huxley, Brave New World, chapter 17
Psychology
Psychology appeared to be a jungle of confusing, conflicting, and arbitrary concepts. These pre-scientific theories doubtless contained insights which still surpass in refinement those depended upon by psychiatrists or psychologists today. But who knows, among the many brilliant ideas offered, which are the true ones? Some will claim that the statements of one theorist are correct, but others will favour the views of another. Then there is no objective way of sorting out the truth except through scientific research.
Raymond Cattell (1965). The Scientific Analysis of Personality, Baltimore, MD: Penguin, p. 14.