There are certain concepts in our culture that seem so natural, so self-evident, that we rarely pause to question them. It is so with feelings and emotions. We speak of our feelings in mechanical terms. We are "bursting with joy" or "filled with rage." We "bottle up" our sadness, "vent" our frustration, or "let off steam." We advise a grieving friend to "let it all out." This language, so common it is invisible, rests on a shared assumption: that an emotion is a substance or an energy - a "great liquid" [1] - that exists inside a container, which is the self. This "hydraulic model" posits that feelings are "heaving and frothing, eager to be let out," and if they are not, the pressure will build until we break.
This is not just folk wisdom; it is a concept embedded in the foundations of Western psychology. The very idea of "catharsis", central to psychoanalysis, is defined as a "purification" or "cleansing". Sigmund Freud and Josef Breuer adapted this concept from Greek drama into a therapeutic method, a "talking cure" designed to "discharge the repressed emotions" that, they argued, were the cause of neurosis. Their model of the psyche was explicitly hydraulic, operating on "psychical pressure" [2]. Later psychoanalytic theory characterized internal emotions as "key 'safety valves of the psyche'".
The persistence of this metaphor, comparisons to steam or psychic energy, is telling. It suggests that Western thought has consistently turned to the language of physics and engineering to explain the inner world. This is likely because the felt experience of an emotion - the physiological arousal of a racing heart, a flushed face, and tensed muscles - feels like pressure. A metaphor chosen to fit this raw sensation (pressure, heat, motion) was, over time, mistaken for an explanation of the emotion itself. This chapter asks how we came to believe that our very selves are pressure cookers. This idea is not a human universal; it is a specific historical construction, one that began not with steam, but with bodily fluids. And it may have been the foundational architecture for the buffered self—creating a boundary that allowed for individual agency.
For nearly two millennia, the Western understanding of emotion was dominated by the theory of the four humors. Originating with Hippocrates and Galen, this system held that the body was a "shell containing four different fluids": blood, phlegm, black bile, and yellow bile. These weren't just biological substances; they were cosmic elements that dictated personality. An excess of hot, dry choler was anger; a surplus of cold, black bile was melancholy.
This model established the self as a container, but a dangerously permeable one, physically determined by the "airs, waters, and places" of the environment. It was against this vulnerability that the Ancient Greek Stoics built the first conceptual wall. In the "porous" world of the Iliad, a rage-filled warrior was "possessed" by a god. To the Stoics, who valued total self-mastery (autarkeia), this was intolerable. If anger is a fluid or a god, I am not responsible. To claim agency, they radically redefined emotion not as a passive suffering (pathos), but as an active judgment (synkatathesis). They argued that fear is not a spirit seizing you, but your mind agreeing to the proposition "this situation is bad." By insisting that emotion was a cognitive act, they partially secularized the internal space to ensure moral responsibility.
If the Stoics drafted the blueprint for the independent self, 17th-century philosophy built the machinery to enforce it. René Descartes' famous dualism was not just an analytical distinction; it was a quarantine designed to fortify the Stoic wall.
By reducing the body to a "machine" and the passions to the mechanical movements of "animal spirits" (fluids), Descartes performed a crucial theological maneuver. He de-spiritualized the body to protect the soul. In his view, if passions were just hydraulic pressure—matter in motion—they could be studied and controlled without threatening the immortal, rational soul. Descartes defined passions as the mind's perception of these mechanical agitations, creating an internal distance where the rational "I" observes its own body's troublesome feelings.
If Descartes built the internal mind, John Locke built the private self. Locke proposed that "every Man has a Property in his own Person," encompassing his mind and labor. This established "privacy as a sphere" [3]-an enclosed, owned space. He argued that the will is moved by a primitive internal feeling of "uneasiness" [4], an internal pressure that drives human action.
This "buffering" reached its peak in the Enlightenment's reaction against religious fervor called then "Enthusiasm" (literally "having a god within"). Emerging science reclassified religious ecstasy not as divine possession, but as "vapors" or "overheated humors"—mere hydraulic malfunctions. This declared that there is no "god within," only gas and fluid, effectively sealing the porous self and rendering the individual safe for a secular society.
However, it is important to note that this "isolated self" was never the only Western voice. As we discussed in an earlier chapter, thinkers like Adam Smith argued for "sympathy" as the glue of society, and later phenomenologists emphasized our shared world. However, it was the specific lineage of Descartes and Locke—centering on the distinct, autonomous self—that captured the medical and scientific imagination, becoming the dominant narrative of the West.
This new, internalized, mechanical model required a new, secular vocabulary. The word we use to describe our feelings, "emotion," is a relatively recent arrival, and its adoption represents a conceptual revolution [5].
The older, dominant term was "passion". Its etymology is revealing, stemming from the Latin pati, meaning "to suffer". A "passion" was something one underwent or suffered, just as a "patient" is one who receives the action of another. It was inherently passive; it was "that which we experience on account of another's action". The term was also deeply tied to theology and morality, most notably in the Passion of Christ or as a "desire to sin". A passion, therefore, was fundamentally relational and moral—an external force acting upon you.
"Emotion," by contrast, entered English in the 1570s from the French émouvoir ("to stir up"), which itself came from the Latin emovere - a combination of e- (out) and movere (to move). Its original meaning was purely physical and mechanical. It described a "social moving, stirring, agitation", a "commotion" among a crowd [6], or even the "emotions of the pulse". It was a word of motion.
Throughout the 17th and 18th centuries, these terms competed. "Passion" often implied a raw, violent state, while "affection" or "sentiment" was used for calmer, more refined feelings. But "emotion" was gaining ground. As historian Thomas Dixon details, it was adopted as a secular category for scientific use. Descartes himself had suggested that "emotions" of the soul was a better, more scientific term than "passions".
By the mid-18th century, "emotion" had migrated from a description of bodily motion to the mental domain [7]. By 1859, it had become the dominant catch-all term, supplanting all others to comprehend "feelings, states of feeling, pleasures, pains, passions, sentiments, affections" [8]. This was not just a linguistic swap. The shift from "passion" (to suffer) to "emotion" (to stir up/move out) moved the concept from the realm of theology and morality to the realm of science and mechanics. "Passion" implies a relational, moral event. "Emotion" implies a mechanical, individual agitation. This new, secular definition of "emotion" aligned perfectly with the emerging concept of the private, autonomous self.