René Descartes

René Descartes

I. Intro

René Descartes was the first modern rationalist, and one of the greatest practitioners of that school of thought. His most important contributions were in the field of mathematics, where he was the first to fuse algebra with geometry, single-handedly inventing the modern field of analytic geometry. This was a revolution in mathematics, and to this day we use a “Cartesian coordinate system,” named for Descartes, to plot algebraic functions.

The great mathematician didn’t stop with the study of numbers – he wanted the same logical, step-by-step thinking to apply in philosophy, indeed in all of life. The ideal philosophy, Descartes reasoned, would be one founded on truths as clear and unassailable as 1+1=2.

This idea sparked the modern school of Rationalism. For generations, philosophers would be captivated by the idea of a perfectly rational system of thought, one with no room for doubt or error. It was a compelling idea. It was also a controversial idea, because it sought to dethrone what had been the center of truth in Europe for centuries: Christianity. Descartes was a Christian and made arguments for the existence of God, but his view was that God had to be the end of the argument, not the beginning. You couldn’t say “God exists, therefore x is true” – you had to say “x is true, therefore God exists.” This put him in conflict with some Christians who wanted to base their religion on faith rather than rationality.


II. Bio

Descartes was born in 1596, the son of a local politician and a mother who died when Descartes was a baby. His father was adamant that his son would get a good education, and sent him off to study with the Jesuit priests at a nearby college. Like many well-educated young men at the time, Descartes studied a wide range of topics, but he focused on law so that he could follow his father’s profession.

As a young law student, Descartes was frustrated by the authoritative, dogmatic style of his professors. He fiercely believed in independent thinking and resented any attempt to corral his mind. So he quit school and started a life of travel – visiting cities all over Europe and finding work as a mercenary and later as a military engineer.

At some point in his mid-twenties, Descartes had an experience that would change not only his own life but the whole history of European philosophy. The story is a little unclear, but it goes something like this: on a cold night in November 1916, the young soldier found himself stationed in a dreary German village with no campfire to keep warm. Just to escape the cold, he shut himself into someone’s furnace and stayed there for hours in the darkness. While sheltering in the furnace, Descartes reports that he had visions: dreams, or possibly hallucinations, that revealed the possibility of a philosophy. He envisioned a school of thought that would encompass philosophy, science, math, indeed all knowledge, and place that knowledge on a foundation so firm that no skepticism could ever shake it.

After emerging from the furnace, Descartes began his writings. It was small stuff at first – after all, he was still living the life of a wandering soldier. Moreover, Descartes had witnessed what happened to Galileo, another mathematician who criticized some powerful interpretations of Christian doctrine, and he didn’t want to suffer the same fate. It was only years later that he began publishing his ideas.

Descartes wrote beautiful, engaging books that proved extremely popular with European nobility. Queens and princesses wrote to him personally, requesting answers to their philosophical questions, and for his remaining years Descartes made a good living as a tutor to Europe’s most powerful women.


III. Descartes’ Ideas

a. The Cogito Argument

Descartes was the one who wrote probably the most famous three words in all of philosophy: cogito ergo sum (“I think, therefore I am”). It’s an enigmatic phrase and doesn’t make much sense out of context. So here’s the context: Descartes was an extreme proponent of skepticism. He believed we should throw out any idea that wasn’t based on absolute certainty, and only reason from principles that we know for sure to be the case.

Skepticism sounds nice in theory, but it quickly gets you into trouble. You can be skeptical about the things your teachers say, or skeptical about a tenet of your religion. But what if you’re skeptical of your own senses? After all, you could be hallucinating or dreaming (remember, Descartes had some personal experience with hallucinations). What if the things you see and hear are just illusions? Worse, what if your whole existence is an illusion? We look around and experience the world, but how can we be sure that our experiences are real? What if you were really just a brain in a vat, being stimulated by electrodes but not really experiencing anything? It’s a disturbing thought, but disproving it isn’t easy! For Descartes, absolute rationality meant we had to take that possibility seriously. We had to find firmer foundations for our reasoning.

After years of intense thought, Descartes finally arrived at one principle that could never be doubted: I think. No skeptic could ever argue the contrary – the very act of doubting your own thoughts is an act of thinking! And that realization led to a second: if I think, then I must exist. A non-existent non-being can’t think, Descartes argued, so we thinkers must be real. Once he had found a solid proof for his own existence, Descartes felt he had a real foundation for developing other philosophical ideas about human nature and the nature of God.

b. Analytic Geometry

It’s hard to imagine science and math without the concept of an x axis and a y axis. Every scatter plot you’ve ever seen, every line graph showing changes in the stock market, every bell curve and parabola – they all come back to the Cartesian coordinate system. Descartes showed that algebraic variables (like x and y) could be represented as points along a number line. Any two variables, therefore, could be defined in relationship to one another using a two-dimensional plot.

Simple as it may seem in hindsight, this was the most revolutionary mathematical idea since the invention of zero. It opened up limitless possibilities for physics (Isaac Newton would elaborate on the Cartesian system with the invention of calculus), and today is a cornerstone of all social and natural science.

IV. Controversies

Within philosophy, psychology, and neuroscience circles, Descartes’ most controversial idea is not analytic geometry or cogito ergo sum. It’s an idea now known as mind-body dualism. For Descartes, mind and body were not only separate entities but totally separate kinds of entities. Ideas had no physical substance, but were ethereal or spiritual things that floated above the physical world of body and matter.

Because of his mind-body dualism, a lot of people assume that Descartes was uninterested in the physical organ of the brain. Not true! Descartes was intensely interested in the study of the human brain – what we would now call neuroanatomy. This interest arose from Descartes’ realization that there was a serious problem in the theory of mind-body dualism. If mind and body were separate, how could they have causal effects on each other? How could my mental experience of a good meal physically cause me to go back and order it again? And how could the smells of my grandmother’s living room physically evoke feelings and memories from childhood?

Descartes speculated that the answer lay in the structure of the human brain. He argued that there was a special organ in the brain that enabled communication between the mental world and spiritual world. That organ, he believed, was the small nub of grey matter now known as the pineal gland. Nearly every brain structure is bilateral, meaning there’s one on each side of the brain. But the pineal gland is a unitary structure found right around the center of the brain. Descartes reasoned that because the pineal gland is unitary, and consciousness is unitary, that the pineal gland must be the seat of consciousness.

We now know that the pineal gland is a relatively simple structure involved in regulating sleep cycles. Moreover, we have a pretty good sense that mental ideas actually do have physical structure – they reside in the electrical activity of the brain and nervous system, and need not be seen as separate entities. Against Descartes’s mind-body dualism, modern science is based on an assumption of monism, in which the mind and the body are really one. The mind, according to this view, is a pattern of physical activity, not a radically separate kind of entity as Descartes argued.

But for others, the physical world of atoms and neurotransmitters is just not enough to account for the richness of human consciousness. Cartesian dualism is still influential with those who find monism unsatisfactory, and the debate between dualists and non-dualists is still one of the major controversies gripping philosophy departments all over the world.


V. Quotes

a. Quote #1

“They [the ancient philosophers] acquired over their thoughts a sway so absolute that they could esteem themselves more rich and happy than other men who, whatever be the favors heaped on them by nature and fortune, if destitute of this philosophy, can never command the realization of all their desires.” (1637)

Like many great philosophers in nearly all traditions, Descartes understood that inner peace begins with letting go of desire. One of his most famous books was an analysis of the passions, or emotions, and their effect on human thought and behavior. Descartes argued that the passions were not an enemy to be subdued through rigid discipline, but a natural force to be mastered through understanding. In this he borrowed from ancient Greek philosophers like Diogenes and Pyrrho, who, in turn, got their ideas from Indian mystics at the time of Alexander the Great. Diogenes and Pyrrho brought back ideas from the growing religion that would become Buddhism and integrated those ideas into classical Greek philosophy. So Descartes’ quote sounds a lot like Buddhism, it’s not by accident!

b. Quote #2

“As soon as I was old enough to pass from under the control of my instructors, I entirely abandoned the study of letters, and resolved no longer to seek any knowledge that could not be found in myself, or in the great book of the world.” (1637)

Here you can almost feel Descartes’ stubborn rejection of authority. As a young man, he gave up studying the law as his father had wanted and took up the life of a soldier. It’s ironic that a man so obsessed with self-knowledge and inner reflection would have taken up a life of so much outward adventure. Compare that to Immanuel Kant, who argued strenuously for the importance of sensory experience, yet never left his home town!


VI. In Pop Culture

a. The Matrix

In The Matrix, the hero discovers early on that his entire reality is just a computer simulation. All of his sensory experiences are being dictated by evil robots in order to keep his physical body under control. The concept is lifted directly from Descartes, who famously struggled to prove that this was not the case, but that it was nonetheless a possibility that philosophy had to account for. The creators of the Matrix deliberately based their world on a sci-fi rendition of Descartes’ thought experiment.

b. Sid Meier’s Alpha Centauri

In in the video game Alpha Centauri, you can encounter this message:

We are all aware that the senses can be deceived, the eyes fooled. But how can we be sure our senses are not being deceived at any particular time, or even all the time? Might I just be a brain in a tank somewhere, tricked all my life into believing in the events of this world by some insane computer? And does my life gain or lose meaning based on my reaction to such solipsism?

Project PYRRHO, Specimen 46, Vat 7. Activity recorded M.Y. 2302.22467. (TERMINATION OF SPECIMEN ADVISED)

The message itself reads exactly like a student encountering Descartes for the first time – grappling with the possibility that our sensory experiences are an illusion, and pondering the ramifications of that possibility. As an added easter egg for philosophy students, look at the name of the project.


When he had his great philosophical visions, Descartes was working as…





Which of these ideas is NOT associated with Descartes?





What is the role of “I think therefore I am” in Descartes’ philosophical system?





Through the ancient philosophers Diogenes and Pyrrho, Descartes’ philosophy of the emotions was influenced by…