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What’s an Internal Sense?

Perhaps the mind and body aren’t as different as we think.

Illustration by Vincent Tsui
Source: Illustration by Vincent Tsui

In 1758, Claude-Adrien Helvétius argued, “All judgment is nothing more than a sensation,” and he was far from alone in this conviction. Eighteenth-century philosophers on all ends of the spectrum, from radical atheists to Christian naturalists, shared the conviction that, strictly speaking, there was no difference between the mental and the physical.

Today, we would consider memory, judgment, and imagination to be affairs of the mind, but according to many eighteenth-century writers, imagination, memory, and their companions weren’t “mental faculties” so much as “internal senses.”

Why did philosophers think of mental operations as “senses,” and what would it change if we were to do the same?

The Break with Descartes

In the seventeenth century, René Descartes boldly proclaimed, “I think therefore I am,” emphasizing that his intellect—separate from the physical senses—was the bearer of ultimate truth.

Descartes left a robust legacy, and over time, the rift between the mind and body became even more starkly drawn. But in the eighteenth century, a number of philosophers started to challenge Descartes’ philosophy. These “sensationalist” philosophers emphasized that humans acquire all knowledge through their senses.

The English philosopher John Locke is often considered the father of Enlightenment sensationalism. Locke argued that humans have no innate ideas and are born with minds like blank slates.

Two main sources of ideas fill this blank slate: sensation and reflection, or the external and internal senses. External senses yield what Locke called “simple ideas.” These are things like the taste of sugar and the coldness of ice. Simple ideas are stored in the mind, and by reflecting on them, humans develop more complex ideas. These include compound ideas (e.g., a sweet and cold popsicle), comparative terms (e.g., bigger and smaller), and abstract concepts (like loyalty and God).

According to Locke, humans cannot conceive of anything outside the realm of sensory experience and reflection. For example, I can only imagine a swimsuit-clad pig eating waffles because I have experience-based ideas of all those components. In other words, there is no mind independent of body.

Condillac’s Statue

In 1754, the French cleric Étienne Bonnot de Condillac gave Locke’s ideas an even simpler turn. In his well-regarded Treatise on Sensations, Condillac claimed that ideas come from only one source: sensation. “Judgment, reflection, the passions, all the operations of the soul are, in a word, nothing more than sensation itself, converted differently,” he asserted.

To walk his readers through this claim, Condillac offered an offbeat coming-of-age narrative. He asked his readers to imagine a senseless marble statue who, over the course of the treatise, would gain the use of one sense at a time.

First, the statue acquired smell. Standing stock-still, the statue had no concepts of shape, color, sound, taste, or feeling, but when he was presented with a rose, he began to form impressions and his whole being was taken up with the scent.

Condillac described this capacity to be consumed by a sensation as “attention,” and once the statue could pay attention to sensations, he began to experience joy and suffering. When he focused on the pleasant smell of the rose, he was filled with pleasure, but if his attention were occupied by a disagreeable odor, he would feel pain.

Now, let’s say you withdrew the rose from the statue’s nose. According to Condillac, the statue’s attention would be redirected to his internal senses. He would begin to think about the sensation of the rose, even though it was no longer present. Voilà! Memory was born.

If the statue were offered a different kind of flower in addition to the rose, he could begin comparing sensations. By recognizing the scents as distinct, the statue’s faculties of judgment and reason would be born. Then, if the statue combined his experience of the rose and the other flower to come up with a new, hybrid scent, then he would be exercising his faculty of imagination.

As the statue gained more and more senses, he was subjected to more and more sensory impressions, and his judgment, imagination, and memory developed in increasingly complex ways.

This thought experiment was designed to show that, no matter how intricate the self may seem, it’s comprised only of modified sensations. For Condillac, the distinction between the mind and body (or internal and external senses) was misleading; they were all part of the same process.

The Embodied Mind

Today, we readily acknowledge that our minds and bodies are connected. Research in psychosomatic medicine and a wide array of other disciplines has focused on their complex relation. But Descartes’ legacy still has a firm hold on the dominant mindset, which often treats the mind and body as separate. Many people readily admit they are linked, but few would suggest they are one and the same.

If we thought of imagination, reason, etc., not as mental faculties, but as internal sensations (in other words, as part and parcel of the body), it would provide us with a more holistic notion of being. The mind and body would no longer be seen as having a cause-and-effect relationship but would, instead, be treated as consubstantial.

In the words of Mark Johnson and Mark Lakoff, who have written about the importance of embodied philosophy, “This is not just the innocuous and obvious claim that we need a body to reason; rather, it is the striking claim that the very structure of reason itself comes from the details of embodiment.” Such reframing unlocks new ways of thinking about human emotions and behavior.

Our world is much more complex than a simple, mind-focused perspective suggests, and the things that we experience have the power to shape us in profound ways. If we openly acknowledged that power and sought to make the most of it, there’s no telling what kinds of feelings, ideas, and new perspectives might be in our grasp.


Condillac, Étienne Bonnot de. Traité des sensations. In Oeuvres completes de Condillac. Paris: Gratiot, Houel, Guillaume, Pougin & Gide, 1798.

Descartes, René. Meditations on First Philosophy, in which the existence of God and the immortality of the soul are demonstrated (1641).

Helvétius, Claude-Adrien. De l’esprit. Paris: Durand, 1758.

Lakoff, George and Mark Johnson. Philosophy in the Flesh: The Embodied Mind and its Challenge to Western Thought. New York: Basic Books, 1999.

Locke, John. An Essay Concerning Human Understanding. Edited by Peter H. Nidditch. Oxford: Oxford University Press, 1979.

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