The Trouble with Defining Energy

“Energy was not out there in the world waiting to be found, a fact of nature finally revealed to human consciousness”

Cara Daggett; The Birth of Energy

When I want to draw out the capaciousness of the term, energy, I ask my students to turn to one another and define it. What is energy? If an individual from an alien planet needed a description of energy as Earthlings know it, what would you tell them? What does it look like? Smell like? Feel like?

As you can imagine, this exercise elicits responses running the gamut from the standard scientific definition of energy (i.e., the ability to perform work) to creative descriptions like, “a child running around on a sugar high,” to a run-down of the chakra system. Regardless of the responses I receive, each group of students contributes a unique collection of definitions. Of course, that is my point: that energy has no singular definition.

Energy is somehow everything and nothing all at once. We learn that everything is energy; that energy is constantly operating in transfer to do things like keep us alive, fuel our machines, heat the planet. But energy is also, ironically, nothing, in that it is almost indescribable. If you attempt to describe what energy is on an elemental level, you will find yourself struggling to some degree. Richard Feynman famously lectured that “we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount” [1]. So, why does energy feel cosmic and universal if it is, in fact, a floating signifier? Where does this term come from, and why does it have so many applications?

On Etymology: Energy Is an Old Word, but Not for Scientists

Energy began as a rhetorical term. We can trace it all the way back to Aristotle, whose energeia (ἐνέργεια) is a combination of the Greek en- (“in”) and -ergon (“work”) [2]. Related to energeia, Aristotle discusses two senses of the Greek term dunamis, which denotes possibility, potential, and the power for change. For Aristotle, there is a kinêsis, or a movement, but also a second meaning of dunamis where the capacity for change exists [3]. You might see here a resemblance to kinetic energy, the energy of an object’s motion, and potential energy, or the energy of position. If so, you’re not wrong, but it would still take centuries before energy found its way into scientific canon.

Aristotle’s energeia is the foundation for the Latin word energia, the French word énergie, and the English word energy. The latter two originated in the sixteenth century and connect work to virtue and goodness [2].

A quick look at the Oxford English Dictionary [4] is also eye-opening. The OED entries for energy include:

  • “With reference to speech or writing: Force or vigour of expression.”
  • “Impressiveness (of an event).”
  • “Exercise of power, actual working, operation, activity; freq. in philosophical language.”

None of these definitions is scientific, and all bear traces of energy’s classical associations. In fact, displacing energy from physics may render the term poetic or peculiarly moralistic. If you’ve ever read William Blake’s The Marriage of Heaven and Hell (1790), you’ll recall that Blake’s Devil has much to say about energy. “Energy is eternal delight,” akin to physical passions. Reason, contrasted with energy, is like intellection. As such, Blake’s Devil comes down firmly on the side of energy [5].

It makes sense that energy is the bedfellow of British Romanticism, the school to which Blake belongs. The Romantics emphasized the power and importance of the individual spirit, and the sublimity of the imagination. Art and poetry originate from this profoundly energetic locus at our core, producing the famous “spontaneous overflow of powerful feelings” that Wordsworth tells us are recollected in tranquility.

But then, where does that leave physics? If Blake’s Devil was contrasting energy and reason at the end of the eighteenth century, when did science appropriate energy for its own agenda?

When and How Did Energy Become a Scientific Term?

Energy had no formal place in physics until the late 1840s, when a group of northern British scientists appropriated the term to unify a range of natural phenomena including heat, light, and electromagnetism. In addition to heuristic unification, these scientists, William Thomson (later Lord Kelvin) and James Clerk Maxwell included, were motivated by a desire to secure institutional authority for their mostly Scottish Presbyterian cohort [6]. They selected the word energy to reorient physics towards the areas of research that studied transfers and potentials. The Newtonian term force, previously used in these cases, was just too limiting to perform the unifying function that energy promised. Thus, energy was scientifically defined as “the ability to perform work.”

Energy now had a scientific home; yet the process of codifying energy wedded the political and theological agendas of these scientists to an emerging concept of how fuel and labor should be managed. In other words, defining energy in science was a political and historically specific act, not purely the outcome of detached, objective investigation. This point should be underscored, and it is the topic of another (future) post.

Because energy had centuries of humanistic usage behind it by the 1840s, these prior associations layered into the nascent science of energy. As you can imagine, this overdetermined thermodynamics during its crucial years of codification, and remains one of the reasons why we, my students included, can find so many definitions for a single term.

But then, why would nineteenth-century scientists want a term that already existed? Why wouldn’t they just create a new one?

There were several justifications for choosing an extant, patently non-scientific word to generate an entirely novel branch of science. Consider the following:

  1. In the mid-nineteenth century, “science” did not remotely resemble the highly-disciplinary, extensively-institutionalized systems we now have under that name. The differentiation of scientific disciplines was a late-nineteenth century affair. For most of the century, and long before, Britain’s educated persons received training in the classics. Garnering authority for a new scientific regime meant convincing an existing intellectual public that such an enterprise mattered. And so, of course, reaching back for energy’s Greco-Roman pedigree made quite a bit of sense.

  2. Recall that energy was also a poetic term. In the nineteenth century, it was not uncommon for scientists to use poetry to shore up support for their scientific arguments. These classically educated “men of science” marshalled the authority of poetry to shift conversations in their own fields. For example, physicist and mathematician James Clerk Maxwell parodied Shelley’s Prometheus Unbound in a paper he delivered at the British Association for the Advancement of Science in 1876. He wrote his own poem about “Energy” dethroning “Force” and shedding its Newtonian limitations [7].

  3. Lastly, the phenomena that the North British scientists tried to unify under the term energy were abstract. Unlike objects dimensionally difficult to access, as in tiny microbes or planets too large and far away, energy’s objects are hard to grasp because they gain no purchase without language and structure to give them form. In other words, you can’t go out and simply find heat, pick it up, bring it home, photograph it, and expect that anyone else doing the same will arrive at the same result. Electromagnetism is even more difficult. One needs language, as well as experimental rigor, to guide these concepts into form. And, because the North British crew wanted to describe the phenomena of conservation and transformation, energy seemed like a perfect word. Remember that, for Aristotle, energeia meant activity, movement, vigor, and dynamicism. It did not conjure up images of stasis.

Transformation is key. The final point I’ll make is that energy is beautifully situated towards the literary because, in both its scientific and its pre-scientific registers, it is fundamentally about transformation. Energy is about preference for movement, which primed its application in governing fuel sources. However, its metaphysical and literary connotations far predate the industrial era. When we consider that energy physics required models, language, and other representational forms to coax energy out of its elusive abstractions, it becomes clear how much the transformations of figurative language complement the energetic transformations of physical phenomena.

Figurative language – like metaphor, metonymy, allegory, and analogy – takes one thing and transforms it into another thing. Using metaphor we might say, “the moon was a face lit from within.” The moon is obviously not a face, yet we turn it into one with language, inspiring imagery that transforms both “the moon” and “a face” on their own into something novel. So, too, does energy describe transfers and relationalities in states of being. When we dive into the weeds of nineteenth-century physics, we find figurative language everywhere. The Victorians applied analogy and metaphor (i.e., the language of transformation) to define and describe energetic phenomena (i.e., the physics of transformation). Take a beat to admire the synergy.

To sum it up, energy began as a humanistic term and accumulated centuries of meaning before physicists appropriated it in the late 1840s for the new science of thermodynamics. At that point, energy’s prior classical associations layered into its new definition as the ability to do work, as well as buttressing a new unifying agenda meant to reorient physics away from Newtonian “force” and towards energy’s potential for conversion, transformation, and also conservation. All this polysemy and layering has left us with an overdetermined term that, astoundingly, manages to extend from the physical to the metaphysical while also escaping an elemental definition. That is why, I argue, deconstructing energy’s capaciousness is worthwhile: because it is not stable. This means that there is enormous potential to dismantle the governing structures of energy that we now find most violent, as in fossil fuel infrastructures. A topic for another time.


[1] Feynman, Richard. The Feynman Lectures on Physics, vol. I: Mainly Mechanics, Radiation, and Heat, Basic Books, 2011.

[2] Daggett, Carla New. The Birth of Energy: Fossil Fuels, Energy, and the Politics of Work, Duke University Press, 2019.

[3] Cohen, Marc S. “Aristotle’s Metaphysics.” The Stanford Encyclopedia of Philosophy. Edited by Edward N. Zalta. Metaphysics Research Lab, Stanford University, 2020.

[4] “energy, n.” OED Online. Oxford University Press, Aug. 2020.

[5] Blake, William. The Marriage of Heaven and Hell. 1790.

[6] Smith, Crosbie. The Science of Energy: A Cultural History of Energy Physics in Victorian Britain. University of Chicago Press, 1998.

[7] Clarke, Bruce. Energy Forms: Allegory and Science in the Era of Classical Thermodynamics. University of Michigan Press, 2001.

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