Mental or psyche energy or activity is the concept of a principle of activity powering the operation of the mind or psyche. Many modern psychologists or neuroscientists would equate it with increased metabolism in neurons of the brain.
Philosophical accounts 
The idea harks back to Aristotle's conception of actus et potentia. "Energy" here used in the literal meaning of "activity" or "operation". Henry More in his 1642 Psychodia platonica; or a platonicall song of the soul defined an "energy of the soul" as including every phantasm of the soul. Julian Sorell Huxley defines "mental energy" as "the driving forces of the psyche, emotional as well as intellectual" (On living in a revolution xv.192, 1944).
Psychoanalytic accounts 
In The Ego and the Id, Freud argued that the id was the source of the personality's desires, and therefore of the psychic energy that powered the mind. Freud defined libido as the instinct energy or force. Freud later added the death drive (also contained in the id) as a second source of mental energy.
In 1928, Carl Jung published a seminal essay entitled "On Psychic Energy". Later, the theory of psychodynamics and the concept of "psychic energy" was developed further by those such as Alfred Adler and Melanie Klein.
Just as physical energy acts upon physical objects, psychological energy would act upon psychological entities, i.e. thoughts. Psychological energy and force are the basis of an attempt to formulate a scientific theory according to which psychological phenomena would be subject to precise laws akin to how physical objects are subject to Newton's laws. This concept of psychological energy is completely separate and distinct from (or even opposed to) the mystical eastern concept of spiritual energy.
In 1874, the concept of "psychodynamics" was proposed with the publication of Lectures on Physiology by German physiologist Ernst Wilhelm von Brücke who, in coordination with physicist Hermann von Helmholtz, one of the formulators of the first law of thermodynamics (conservation of energy), supposed that all living organisms are energy-systems also governed by this principle. During this year, at the University of Vienna, Brücke served as supervisor for first-year medical student Sigmund Freud who adopted this new "dynamic" physiology. In his Lectures on Physiology, Brücke set forth the then-radical view that the living organism is a dynamic system to which the laws of chemistry and physics apply.
Neuroscientific accounts 
Mental energy has been repeatedly compared to or connected with the physical quantity energy.
Studies of the 1990s to 2000s (and earlier) have found that mental effort can be measured in terms of increased metabolism in the brain. The modern neuroscientific view is that brain metabolism, measured by functional magnetic resonance imaging or positron emission tomography is a physical correlate of mental activity.
See also 
- Ego, superego, and id
- Energy (esotericism)
- Energy psychology
- Death drive
- Hall, Calvin S.; Nordby, Vernon J. (1999). A Primer of Jungian Psychology. New York: Meridian. ISBN 0-452-01186-8.
- Hall, Calvin, S. (1954). A Primer in Freudian Psychology. Meridian Book. ISBN 0-452-01183-3.
- Bowlby, John (1999). Attachment and Loss: Vol I, 2nd Ed. Basic Books. pp. 13–23. ISBN 0-465-00543-8.
- Benton, D., Parker, P. Y., & Donohoe, R. T. (1996). The supply of glucose to the brain and cognitive functioning. Journal of Biosocial Science, 28, 463–479. Fairclough, S. H., & Houston, K. (2004). A metabolic measure of mental effort. Biological Psychology, 66, 177-190. Gailliot, M.T., Baumeister, R.F., et al. (in press). Self-control relies on glucose as a limited energy source: Willpower is more than a metaphor. Journal of Personality and Social Psychology.
Further reading 
- Jung, C.G. (1960). On the Nature of the Psyche. Princeton: Princeton University Press. ISBN 0-691-01751-4.
- Laplanche, J.|Jean Laplanche and Pontalis, J.B. (1974). The Language of Psycho-Analysis. Trans. Donald Nicholson-Smith. New York: W. W. Norton & Company, 1974.