ABOVE: Modified from © ISTOCK.COM, Anthony SEJOURNE

Imagination is the faculty of the mind that adds life and color to our existence, transforming us into the infinitely perplexing, captivating creatures we are—creatures who err, transgress, and suffer, but also fancy, organize, and invent. According to French philosopher Michel de Montaigne, imagination sculpts every aspect of our experience, from the most intimate to the most public. It is by its grace that we anticipate events in the offing, empathize with others when they are in pain, and even fool ourselves into believing that an ersatz treatment has cured us simply because a physician prescribed it—the notorious placebo effect. It is by the grace of imagination, too, that we dream. As Montaigne says in his 16th-century essays, to dream is to “[rove] in the vague expanse of the imagination.” 

Even though he is widely considered a humanist, Montaigne recognized that humans are not the only creatures endowed with this quasi-magical faculty. “Even brute beasts,” he writes, “are subject to the force of imagination as well as we; witness dogs, who . . . bark and tremble and startle in their sleep; so horses will kick and whinny in their sleep.” These facts can only mean the following, he concludes: When sleep overcomes them, these creatures’ minds are engrossed in the act of building dreamworlds, imaginary mental holograms of reality itself. 

Today, empirical research on animal sleep suggests that the prodigious French essayist was correct in thinking that other animals are also subject to “the force of imagination,” at least as far as dreaming is concerned. In my new book, When Animals Dream: The Hidden World of Animal Consciousness, I examine this research and take a close look at its scientific, philosophical, and ethical implications. 

While most of this research is quite recent, some of it is already half a century old. For instance, in the late 1960s, the French physician Yves Ruckerbusch, who directed the Laboratory of Physiology and Pharmacodynamics at l’École nationale vétérinaire de Toulouse, wondered what happens to horses (Equus caballus) when night falls. Do they sleep like we do? Is their sleep cycle divided into different phases like ours is? And if so, do they experience that peculiar phase of the human sleep cycle known as rapid eye movement (REM) sleep during which dreams generally occur? 

Using EEG technology, Ruckerbusch discovered not only that horses experience their own version of REM sleep, but that during this phase of their sleep cycle, they show levels of neuronal and behavioral activation reminiscent of waking life. That is to say, during REM sleep their brains, and to some extent their bodies, behave in ways normally affiliated with waking experience, even when they give every indication of being asleep (closed eyelids, lowered body temperature, etc.). For example, their brains yielded an electrical signature indistinguishable from that of the waking state while their bodies displayed what Ruckerbusch called “spectacular motor manifestations,” including movements of the eyes, limbs, and even nasolabial muscles. At one moment, the horses in his study were perfectly still; the next, their bodies put on a spectacle for all to see—a rapid eye movement here, a nasal juddering there, a kick of the tarsus over there. 

Ruckerbusch does not use the term “dream,” but this is a clear way of making sense of what was happening to his four-legged research subjects. In all likelihood, these horses were having a dream, roving in that “vague expanse of the imagination” to which Montaigne called attention. Ruckerbusch even points out that the horses’ neuronal and behavioral displays coincided with physiological changes that usually indicate a dream sequence, especially accelerated respiration and heart rates.

Can we imagine a better testament to the power of the animal mind than its ability to invent for itself alternative realities during sleep?

Since the 1970s, Ruckerbusch’s findings have been replicated in scores of other animals, including dogs, rats, elephants, primates, birds, and cephalopods. In 2010, for instance, researchers from the University of Minnesota found that sleeping rats can mentally visualize scenes of behavioral sequences set in places without a corresponding location in the real world, which is to say, places that exist only in their imaginations. This means that Homo sapiens are neither the only beings who dream nor the only ones who move through the world guided by what Aristotle calls the phantastikon, the soul’s capacity for sensible imagining—a capacity that, like Montaigne, Aristotle occasionally extends to “beasts.” 

In When Animals Dream, I argue that the mere fact that animals dream poses a formidable challenge to that bastion of traditionalism that is the human-animal divide, raising provocative ethical questions about the status of animals as moral subjects toward whom we have urgent and inexorable ethical obligations. This fact also frustrates the common view that only humans are “cognitively free” because only we can liberate ourselves from our immediate surroundings through acts of imagination, while the rest of the animals remain forever trapped in the here and now. The dreams of other species elegantly refute this claim by suggesting that non-human life forms also stage mental escapes from reality during sleep, fleeing from the physical world that surrounds them and charging into an imaginary world of subjective make-believe. 

Can we imagine a better testament to the power of the animal mind than its ability to invent for itself alternative realities during sleep? Can we envisage a more inspiring incarnation of imagination itself?

David M. Peña-Guzmán is an associate professor of humanities and liberal studies at San Francisco State University. Read an excerpt of When Animals Dream.