Freud conceived of the mind as having only a fixed amount of psychic energy, or libido. Though the word libido has since acquired overt sexual implications, in Freud's theory it stood for all psychic energy. This energy fueled the thought processes, perception, imagination, memory, and sexual urges. In Freud's theory, the mind, like the universe, could neither create nor destroy energy, but merely transfer it from one form or function to another. Because scope of the mind's capabilities was thus limited by the amount of psychic energy freely available, any process or function of the mind which consumed excess energy debilitated the ability of the mind to function normally. Repression, he held, demanded significant amounts of energy to maintain; even then, a repressed thought might come perilously close to becoming conscious, only to be redirected or defended against by a defense mechanism. As well, a fixation on a past psychosexual stage of development could permanently sap this libidal energy, causing, in the extreme cases, neuroses or worse.

The dynamic interaction between the id, ego and superego, with each contending for as much libidal energy as possible, illustrates the importance of the functions of the mind. A man who invests most of his libidal energy into the cravings of his id will act and live much differently than the man whose guilt-inspiring superego consumes most of his libidal energy. This constantly changing balance and interaction between the various functions of the mind, in Freud's theory, determines personality.

Last modified 1998