In psychological thermodynamics, the ego is a term representing a state of the mind associated with various energy levels or levels of conscious activity.

History
It seems that English scientist Herbert Spencer was one of the first to connect the principle of energy to the term ego when in his 1872 edition Principles of Psychology (first published in 1855) he stated in his explanation of cognition that: [1]

“Sets of manifestations [are] controllable by an energy every welling up within, are grouped together as an ego, and that set of manifestations which, not being thus controllable, originate the consciousness of an outer energy or non-ego.”

This type of logic was later introduced more elaborately by Austrian psychologist Sigmund Freud in his 1923, non-classic, The Id and the Ego, in which he divided the mind into the id, the unconscious source of drive and energy, the ego, the partly-conscious state of the active mind in its present plan, and the super-ego, a residue of the earliest object-choices of the id (stemming from the pleasure principle) representing an energetic-formation against those choices. [2]

Various aspects of Freud’s three-part energy model of these various ego states or parts of the mind, were taken later by those as Carl Jung, Eric Berne, Mihály Csíkszentmihályi, and Francisco Tellez, among numerous others.

References
1. Spencer, Herbert. (1906). Principles of Psychology, Vol II, Installment No. 33 (pp. 481-560, esp. pgs. 501) Ed. 3. D. Appleton and Co.
2. Freud, Sigmund (1923). The Ego and the Id (pgs. 4-5). W.W. Norton & Company.

External links
● Id, ego, and super-ego – Wikipedia.