Animal spirits is a term used by John Maynard Keynes in his 1936 book The General Theory of Employment, Interest and Money to describe the instincts, proclivities and emotions that seemingly influence human behavior, which can be measured in terms of consumer confidence. The concept has also been discussed in behavioral economics and social psychology, where it is used to explain how collective emotions and instincts can drive decision-making beyond rational calculation.

The original passage by Keynes reads:

Even apart from the instability due to speculation, there is the instability due to the characteristic of human nature that a large proportion of our positive activities depend on spontaneous optimism rather than on a mathematical expectation, whether moral or hedonistic or economic. Most, probably, of our decisions to do something positive, the full consequences of which will be drawn out over many days to come, can only be taken as a result of animal spirits – of a spontaneous urge to action rather than inaction, and not as the outcome of a weighted average of quantitative benefits multiplied by quantitative probabilities.

The notion of animal spirits has been described by René Descartes, Isaac Newton, and other scientists as how the notion of the vitality of the body is used.^{[citation needed]}

In one of his letters about light, Newton wrote that animated spirits live in "the brain, nerves, and muscles, [which] may become a convenient vessel to hold so subtle a spirit." These spirits, as described by Newton, are animated spirits of an ethereal nature, relating to life in the body. Later, it became a concept that acquired a psychological content, but was always thought of in connection with the life processes of the body. Therefore, they retained a lower overall animal status.

William Safire explored the origins of the phrase in his 2009 article "On Language: 'Animal Spirits'":

The phrase that Keynes made famous in economics has a long history. "Physitions teache that there ben thre kindes of spirites", wrote Bartholomew Traheron in his 1543 translation of a text on surgery, "animal, vital, and natural. The animal spirite hath his seate in the brayne ... called animal, bycause it is the first instrument of the soule, which the Latins call animam." William Wood in 1719 was the first to apply it in economics: "The Increase of our Foreign Trade...whence has arisen all those Animal Spirits, those Springs of Riches which has enabled us to spend so many millions for the preservation of our Liberties." Hear, hear. Novelists seized its upbeat sense with enthusiasm. Daniel Defoe, in "Robinson Crusoe": "That the surprise may not drive the Animal Spirits from the Heart." Jane Austen used it to mean "ebullience" in "Pride and Prejudice": "She had high animal spirits." Benjamin Disraeli, a novelist in 1844, used it in that sense: "He...had great animal spirits, and a keen sense of enjoyment."

Thomas Hobbes used the phrase "animal spirits" to refer to passive emotions and instincts, as well as natural functions like breathing.