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.
For nearly two millennia, the Western understanding of personality and emotion was dominated by the theory of the four humors. This system, originating with Greek physicians like Hippocrates and codified by Galen, underpinned European medicine until at least the 1700s. It held that the human body was, as one writer put it, "a shell containing four different humours, or fluids": blood, phlegm, black bile, and yellow bile.
These fluids were not just biological products; they were the microcosm of the universe. They corresponded directly to the four elements (air, water, earth, fire), the four qualities (hot, moist, cold, dry), and the four seasons. A person's temperament was determined by their dominant humor. An excess of:
The crucial point of this model is that "emotional states are physically determined". The "core passions of anger, grief, hope, and fear" were "bred" by these internal fluids. Anger was an excess of hot, dry choler. Melancholy was a surplus of cold, dry black bile.
This ancient theory established the fundamental precedent for the modern individualistic view: it framed the human being as an internal, closed, homeostatic system. Health, both physical and mental, was a matter of internal balance. Consequently, the management of the passions was not primarily a social or ethical task, but a physical and individual one. Treatments like bloodletting or dietary changes were forms of fluid management, attempts to rebalance the internal vessel. The cause of an imbalance might be external (like the "airs, waters, and places" Hippocrates wrote about), but the passion itself was located, defined, and treated inside the individual's body. This created a 2,000-year-old intellectual habit of seeing the self as a container that must be internally managed—the very first draft of the pressure-vessel self.
If humoralism provided the concept of internal fluids, 17th-century philosophy built the container to hold them. It was in this period that the modern, private, individual self was architected.
The first major shift came from René Descartes. His famous mind-body dualism split the human into two distinct substances: the mind (an immaterial, rational consciousness) and the body (a material, passive "machine") [3]. In this new system, the passions were a problem. Descartes defined them as the mind's perception of the body's mechanical agitations. They were "coarse and often-troublesome" feelings that took root in the body, distinct from the "more subtle 'Passions of the Soul'" that arose from the intellect. This created what one medical historian calls a "long medical tradition of separating 'rational' consciousness from emotions". It provided a mechanistic account of feeling [4], creating an internal distance where the rational "I" observes its own body's troublesome passions.
If Descartes built the internal mind, John Locke built the private self. Locke proposed the radical concept that "every Man has a Property in his own Person". This "property" included not just one's body and labor, but one's life, liberty, and mind. This idea, foundational to liberal thought, established "the conception of privacy as a sphere" [5]—an enclosed, owned space that cannot be entered "without his consent".
Finally, Locke provided a new internal engine to drive this private self. He argued that the will is not moved by rational assessment of the good, but by a more primitive internal feeling: "uneasiness" [6]. This "uneasiness," the discomfort of a desire or absence, is the internal motivator that drives all human action.
This 17th-century project created the architecture for the modern emotion. The feeling now lives inside the mechanical body (from Descartes) and is owned by the private self (from Locke). "Uneasiness" is the internal pressure that moves this new, private machine. This is the critical moment when emotion becomes truly, conceptually individualistic. My feelings are my property, held within my private sphere. It is a model that, by its very design, disregards "interpersonal... reactions to disease" [7] because the self is now a self-contained, self-owned entity.