Three or One or None? A Short History of the "Soul"
Is there such a thing as a soul? And how Leibniz stumbled upon the unconscious.
Updated May 30, 2026 | Reviewed by Tyler Woods
For Aristotle (384-322 BCE), all living things had a vegetative or nutritive soul; animals also had a sensitive soul; and humans, on top of that, had a rational soul. As a result, medieval theologians and philosophers debated whether humans had a plurality of souls.
To pluralists such as Archbishop of Canterbury Robert Kilwardby (c. 1215-1279), the vegetative, sensitive, and rational souls were distinct entities stacked within the human body. For Kilwardby, this served to explain how Christ’s body remained holy in the tomb after his human soul had departed.
The pluralists were fiercely opposed by unitists such as St Thomas Aquinas (1225-1274), who argued that a person with multiple souls would be no more than a bundle of parts, rather than a single, unified substance. For a short time after his death, Aquinas's single soul "heresy" was banned in Paris and Oxford. Following centuries of debate, the unitist view, of course, came to prevail.
The philosopher René Descartes (1596-1650) completely changed the conversation. To uphold his dualism of mind and body and defend the immortality of the human soul, Descartes argued that only humans (and higher beings such as angels) possessed both a physical body and an immaterial soul. Being the source of thought and reason, this immaterial soul qualified human beings for things like heaven and eternal life. All other living things—plants and animals—were soulless, and functioned like complex machines.
Animals, claimed Descartes, are mere automata, whose functions “follow from the mere arrangement of the machine’s organs every bit as naturally as the movements of a clock or other automaton follow from the arrangement of its counterweights and wheels.” For Descartes, it is not only that animals do not reason, but also that they do not feel, perceive, or sense. Clearly, he did not have a cat. According to lore, he once threw a cat out of a window to demonstrate that the poor thing lacked consciousness.
Although French, Descartes spent the greater part of his adult life in the relative safety of the Dutch Republic. There, he met a servant called Helena, who became the mother of his illegitimate daughter, Francine (b. 1635), whom he passed off as his niece. When Francine died, aged five, of scarlet fever, he called it the greatest sorrow of his life—even stating that it was not unmanly to cry. Later, a legend arose that he constructed an automaton in Francine's likeness.
How Leibniz Stumbled Upon the Unconscious
In the early 1700s, the polymath Gottfried Leibniz (1646-1716) investigated and documented a famous “talking dog” in the village of Zeitz, Saxony. He concluded that the dog was not in fact “talking,” but merely echoing the sounds of his master.
Even so, Leibniz rejected Descartes’s idea of animals as soulless machines. Relying heavily on the Aristotelian concept of entelechy (the vital force that drives an organism), he returned their sensitive soul to the animals. Still, animals lack a rational soul, and act purely empirically, that is, on the basis of experience. Instead of reasoning or calculating, they merely associate ideas—as do we, most of the time. It is not all that often that we actually reason, and some of us, it seems, never do.
Whereas Aristotle had made self- nutrition the fundamental characteristic of living things, Leibniz opted for perception and appetition. Leibniz distinguished between perception and its heightened form, apperception, that is, conscious perception. In bare monads, perception is diffuse and unconscious , but in animals and humans, it is conscious and focused. Bare monads, which ground inanimate bodies, are as in a deep sleep or stupor, and unconscious of their perceptions.
For Leibniz, humans, when unconscious or in a deep sleep, are as bare monads. That a loud noise can rouse us from a deep sleep indicates that, even then, we are perceiving, though unaware of doing so.
In his Discourse on Metaphysics (1686), Leibniz wrote:
… even though our senses are related to everything, it is impossible for our soul to attend to everything in particular; that is why our confused sensations are the result of a truly infinite variety of perceptions. This is almost like the confused murmur coming from the innumerable set of breaking waves heard by those who approach the seashore…
In other words, there are two levels of consciousness, one where we register stimuli, and the other where the most salient of these stimuli, or the mean of these stimuli, are brought to our conscious attention . This notion of petites perceptions (“small perceptions”, or unconscious perceptions) anticipates Schopenhauer and Freud .
Humans, unlike animals, are able to reflect upon their perceptions to derive the notions of the “I” or self, and of God. This sort of meta-reflection ("reflection upon reflection") calls to mind Aristotle’s description, in Book Lambda of the Metaphysics , of the activity of God, which, he says, consists in “a thinking upon thinking”. It is this notion, or meta-notion, of the self, subsisting through time, that makes us moral beings, and susceptible to punishment and reward.
In returning their soul to the animals, Leibniz had to explain how their minds worked without relying on conscious reason. In so doing, he shattered the Cartesian assumption that mind and consciousness are the same, effectively discovering the unconscious mind. In his own words, “the difference between intelligent substances and substances that have no intelligence at all is just as great as the difference between a mirror and someone who sees.”
Neel Burton is the author of the newly published The German Greeks: German Philosophy and the German Philosophers .
René Descartes, Treatise on Man (1664, posthumously).
Gottfried Leibniz, Discourse on Metaphysics (1846, posthumously).
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Neel Burton, M.D. , is a psychiatrist, philosopher, and writer who lives and teaches in Oxford, England.
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