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.
Health psychology is the study of psychological and behavioral processes in health, illness, and healthcare. It is concerned with understanding how psychological, behavioral, and cultural factors contribute to physical health and illness. Psychological factors can affect health directly. For example, chronically occurring environmental stressors affecting the hypothalamic–pituitary–adrenal axis, cumulatively, can harm health. Behavioral factors can also affect a person's health. For example, certain behaviors can, over time, harm (smoking or consuming excessive amounts of alcohol) or enhance health (engaging in exercise). Health psychologists take a biopsychosocial approach. In other words, health psychologists understand health to be the product not only of biological processes (e.g., a virus, tumor, etc.) but also of psychological (e.g., thoughts and beliefs), behavioral (e.g., habits), and social processes (e.g., socioeconomic status and ethnicity).
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.
There are three wicks you know to the lamp of a man's life: brain, blood, and breath. Press the brain a little, its light goes out, followed by both the others. Stop the heart a minute, and out go all three of the wicks. Choke the air out of the lungs, and presently the fluid ceases to supply the other centres of flame, and all is soon stagnation, cold, and darkness.
Oliver Wendell Holmes, Sr., The Professor at the Breakfast Table (1859), XI.
There are more disorders of the mind than of the body, and they are of a more dangerous nature.
Marcus Tullius Cicero, Tusculanae Disputationes (c. 45 BCE), Book III, Section III, p 93, translated by Charles Duke Yonge (1877 edition)
The popular medical formulation of morality that goes back to Ariston of Chios, "virtue is the health of the soul," would have to be changed to become useful, at least to read: "your virtue is the health of your soul." For there is no health as such, and all attempts to define a thing that way have been wretched failures. Even the determination of what is healthy for your body depends on your goal, your horizon, your energies, your impulses, your errors, and above all on the ideals and phantasms of your soul. Thus there are innumerable healths of the body; and the more we allow the unique and incomparable to raise its head again, and the more we abjure the dogma of the "equality of men," the more must the concept of a normal health, along with a normal diet and the normal course of an illness, be abandoned by medical men. Only then would the time have come to reflect on the health and illness of the soul, and to find the peculiar virtue of each man in the health of his soul.
Friedrich Nietzsche, The Gay Science, § 120 “Health of the Soul”