17 Things to Know About René Descartes

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The French polymath René Descartes (1596-1650) lived after the Renaissance, but he personified that age's interest in mathematics, philosophy, art, and the nature of humanity. He made numerous discoveries and argued for ideas that people continue to grapple with. (His dualist distinction between mind and the brain, for example, continues to be debated by psychologists.) Get to know him better!

1. NOBODY CALLED HIM RENÉ.

Descartes went by a nickname and often introduced himself as “Poitevin” and signed letters as “du Perron.” Sometimes, he went so far to call himself the “Lord of Perron.” That’s because he had inherited a farm from his mother’s family in Poitou, in western France.

2. SCHOOL MADE HIM FEEL DUMBER.

From the age of 11 to 18, Descartes attended one of the best schools in Europe, the Jesuit College of Henry IV in La Flèche, France. In his later work Discourse on the Method, Descartes wrote that, upon leaving school, “I found myself involved in so many doubts and errors, that I was convinced I had advanced no farther in all my attempts at learning, than the discovery at every turn of my own ignorance."

3. HIS DAD WANTED HIM TO BE A LAWYER.

Descartes’s family was chock-full of lawyers, and the budding intellectual was expected to join them. He studied law at the University of Poitiers and even came home with a law degree in 1616. But he never entered the practice. In 1618, a 22-year-old Descartes enlisted as a mercenary in the Dutch States Army instead. There, he would study military engineering and become fascinated with math and physics.

4. HE CHANGED CAREER PATHS THANKS TO A SERIES OF DREAMS.

In 1618, the Emperor of the Holy Roman Empire, Ferdinand II, attempted to impose Catholicism on anybody living within his domain. The result of this policy would be the Thirty Years' War. It would also prompt Descartes, a Catholic, to switch allegiances to a Bavarian army fighting for the Catholic side. But on his travels, he stopped in the town of Ulm. There, on the night of November 10, he had three dreams that convinced him to change his life’s path. “Descartes took from them the message that he should set out to reform all knowledge,” philosopher Gary Hatfield writes in the Stanford Encyclopedia of Philosophy.

5. HE COULD BE EASILY DISTRACTED BY BRIGHT AND SHINY OBJECTS.

In 1628, Descartes moved to the Netherlands and spent nine months doggedly working on a theory of metaphysics. Then he got distracted. In 1629, a number of false suns—called parhelia, or “sun dogs”—were seen near Rome. Descartes put his beloved metaphysics treatise on the back burner and devoted his time to explaining the phenomenon. It was a lucky distraction: It led to his work The World, or Treatise on Light.

6. HE LAID THE GROUNDWORK FOR ANALYTIC GEOMETRY ...

In 1637, Descartes published his groundbreaking Discourse on the Method, where he took the revolutionary step of describing lines through mathematical equations. According to Hatfield, “[Descartes] considered his algebraic techniques to provide a powerful alternative to actual compass-and-ruler constructions when the latter became too intricate.” You might have encountered his system in high school algebra: They’re called Cartesian coordinates.

7. ... AND THE REST OF WESTERN PHILOSOPHY.

Everybody knows Descartes for his phrase Cogito, ergo sum (which originally appeared in French as "Je pense, donc je suis"), or "I think, therefore I am." The concept appeared in many of his texts. To understand what it means, some context is helpful: At the time, many philosophers claimed that truth was acquired through sense impressions. Descartes disagreed. He argued that our senses are unreliable. An ill person can hallucinate. An amputee can feel phantom limb pain. People are regularly deceived by their own eyes, dreams, and imaginations. Descartes, however, realized that his argument opened a door for "radical doubt": That is, what was stopping people from doubting the existence of, well, everything? The cogito argument is his remedy: Even if you doubt the existence of everything, you cannot doubt the existence of your own mind—because doubting indicates thinking, and thinking indicates existing. Descartes argued that self-evident truths like this—and not the senses—must be the foundation of philosophical investigations.

8. HE'S THE REASON YOUR MATH TEACHER MAKES YOU CHECK YOUR WORK.

Descartes was obsessed with certainty. In his book Rules for the Direction of the Mind, “he sought to generalize the methods of mathematics so as to provide a route to clear knowledge of everything that human beings can know,” Hatfield writes. His advice included this classic chestnut: To solve a big problem, break it up into small, easy-to-understand parts—and check each step often.

9. HE LIKED TO HIDE.

Descartes had a motto, which he took from Ovid: “Who lives well hidden, lives well.” When he moved to the Netherlands, he regularly changed apartments and deliberately kept his address a secret. Some say it's because he simply desired privacy for his philosophical work, or that he was avoiding his disapproving family. In his book titled Descartes, philosopher A. C. Grayling makes another suggestion: "Descartes was a spy."

10. HE WASN'T AFRAID OF CRITICS. IN FACT, HE RE-PUBLISHED THEM.

When Descartes was revising his Meditations on First Philosophy [PDF], he planned to send the manuscript to “the 20 or 30 most learned theologians” for criticism—a sort of proto-peer review. He collected seven objections and published them in the work. (Descartes, of course, had the last word: He responded to each criticism.)

11. HE COULD THROW SHADE WITH THE BEST OF THEM.

In the 1640s, Descartes’s pupil and friend Henricus Regius published a broadsheet that distorted Descartes’s theory of the mind. (Which, put briefly, posits that the material body and immaterial mind are separate and distinct.) The two men had a falling out, and Descartes wrote a rebuttal with a barbed title that refused to even acknowledge Regius’s manifesto by name: It was simply called “Comments on a Certain Broadsheet.”

12. HE NEVER BELIEVED MONKEYS COULD TALK.

There’s a “fun fact” parading around that suggests Descartes believed monkeys and apes could talk. He believed no such thing. According to the Stanford Encyclopedia of Philosophy, Descartes denied that animals were even conscious, let alone capable of speech. The factoid comes from a misreading of a letter Descartes had written in 1646, in which he attributed the belief to “savages.”

13. HE TOTALLY HAD THE HOTS FOR CROSS-EYED WOMEN.

In a letter to Queen Christina of Sweden, Descartes explained that he had a cross-eyed playmate as a child. “I loved a girl of my own age ... who was slightly cross-eyed; by which means, the impression made in my brain when I looked at her wandering eyes was joined so much to that which also occurred when the passion of love moved me, that for a long time afterward, in seeing cross-eyed women, I felt more inclined to love them than others.”

14. WHEN HE MET BLAISE PASCAL, THEY GOT INTO AN ARGUMENT ... ABOUT VACUUMS.

In 1647, a 51-year-old Descartes visited the 24-year-old prodigy and physicist Blaise Pascal. Their meeting quickly devolved into a heated argument over the concept of a vacuum—that is, the idea that air pressure could ever be reduced to zero. (Descartes said it was impossible; Pascal disagreed.) Later, Descartes wrote a letter that, depending on your translation, said that Pascal had “too much vacuum in his head.”

15. HIS WORK WAS BANNED BY THE CATHOLIC CHURCH.

Back in the late 1630s, the theologian Gisbert Voetius had convinced the academic senate of the University of Utrecht to condemn the philosopher’s work. (Descartes was Catholic, but his suggestion that the universe began as a “chaotic soup of particles in motion,” in Hatfield's words, was contrary to orthodox theology.) In the 1660s, his works were placed on the church’s Index of Prohibited Books.

16. HE REGULARLY SLEPT UNTIL NOON (AND TRYING TO BREAK THE HABIT MIGHT HAVE KILLED HIM).

Descartes was not a morning person. He often snoozed 12 hours a night, from midnight until lunchtime. In fact, he worked in bed. (Sleep, he wisely wrote, was a time of “nourishment for the brain.”) But according to the Journal of Historical Neuroscience, he may have had a sleep disorder that helped end his life. A year before his death, Descartes had moved to Stockholm to take a job tutoring Queen Christina, a devoted early-riser who forced Descartes to change his sleep schedule. Some believe the resulting sleep deprivation weakened his immune system and eventually killed him.

17. HIS SKELETON HAS TRAVELED FAR AND WIDE.

Descartes died in Stockholm in 1650 and was buried outside the city. Sixteen years later, his corpse was exhumed and taken to Paris. During the French Revolution, his bones were moved to an Egyptian sarcophagus at the Museum of French Monuments. Decades later, when plans were made to rebury Descartes in an abbey, officials discovered that most of his bones—including his skull—were missing. Shortly after, a Swedish scientist discovered a newspaper advertisement attempting to sell the polymath’s noggin [PDF]. Today, his head is in a collection at the Musée de l’Homme in Paris.

10 Juicy Facts About Leeches

Ian Cook
Ian Cook

Leeches get a bad rap, but they’re actually pretty cool once you get to know them—and we're finding out more about them, even today. Recently, a team led by Anna Phillips, curator of parasitic worms at the Smithsonian National Museum of Natural History, discovered a new species of medicinal leech (pictured above) in a Maryland swamp. We asked parasite expert and curator at the American Museum of Natural History Mark E. Siddall to share some surprising facts about the worms we love to hate. 

1. Not all leeches suck blood.

Hematophagous, or blood-feeding, species are only one type of leech. “The vast majority of species are [hematophagous],” Siddall tells Mental Floss, “but it depends on the environment. In North America, there are probably more freshwater leeches that don’t feed on blood than there are blood-feeders.” And even among the hematophagous species, there are not too many who are after you. “Very few of them are interested in feeding on human blood,” Siddall says. “Certainly they’ll do it, if they’re given the opportunity, but they’re not what they’re spending most of their time feeding on.” 

2. Leeches are everywhere.

Japanese leech on a log
Pieria, Wikimedia Commons // Public Domain

“Every continent on the planet has leeches, with the exception of Antarctica,” Siddall says. “And even then there are marine leeches in Antarctic waters.” Humans have co-existed with leeches for so long, according to Siddall, that just about every language has a word for leech. 

3. Leeches have made a comeback in medicine.

Bloodletting for bloodletting’s sake has fallen out of favor with Western physicians, but that doesn’t mean medicinal leeches are enjoying a cushy retirement. Today, surgeons keep them on hand in the operating room and use them as mini-vacuums to clean up blood. “That is a perfectly sensible use of leeches,” Siddall says. Other uses, though, are less sensible: “The more naturopathic application of leeches in order to get rid of bad blood or to cure, I don’t know, whatever happens to ail you, is complete hooey,” he says. How on Earth would leeches take away bad blood and leave good blood? It’s silly.” 

4. Novelist Amy Tan has her own species of leeches.

Land-based leeches made an appearance in Tan’s 2005 book Saving Fish from Drowning, a fact that instantly put the author in leech researchers’ good graces. “There are not a lot of novels out there with terrestrial leeches in them,” Siddall says. So when he and his colleagues identified a new species of tiny terrestrial leeches, they gave the leech Tan’s name. The author loved it. “I am thrilled to be immortalized as Chtonobdella tanae,” Tan said in a press statement. “I am now planning my trip to Queensland, Australia, where I hope to take leisurely walks through the jungle, accompanied by a dozen or so of my namesake feeding on my ankles.”

5. Leeches can get pretty big.

The giant Amazon leech (Haementeria ghilianii) can grow up to 18 inches and live up to 20 years. And yes, this one’s a blood-feeder. Like all hematophagous species, H. ghilianii sticks its proboscis (which can be up to 6 inches long) into a host, drinks its fill, and falls off. Scientists thought the species was extinct until a zoologist found two specimens in the 1970s, one of whom he named Grandma Moses. We are not making this up.

6. Leeches make good bait.

Many walleye anglers swear by leeches. “A leech on any presentation moves more than other types of live bait," pro fisher Jerry Hein told Fishing League Worldwide. "I grew up fishing them, and I think they're the most effective live bait around no matter where you go." There’s an entire leech industry to provide fishers with their bait. One year, weather conditions kept the leeches from showing up in their typical habitats, which prevented their collection and sale. Speaking to CBS news, one tackle shop owner called the absence of leeches “the worst nightmare in the bait industry.”

7. Leech scientists use themselves as bait.

Siddall and his colleagues collect and study wild leeches. That means hours of trekking through leech territory, looking for specimens. “Whether we’re wandering in water or traipsing through a bamboo forest,” Siddall says, “we are relying on the fact that leeches are attracted to us.” Do the leeches feed on them? “Oh my god, yes. We try to get them before they feed on us … but sometimes, obviously, you can’t help it.”

8. Leech sex is mesmerizing.

Like many worms, leeches are all hermaphroditic. The specifics of mating vary by species, but most twine themselves together and trade sperm packets. (The two leeches in the video above are both named Norbert.)

9. Some leech species make surprisingly caring parents. 

“There’s a whole family of leeches that, when they lay their eggs, will cover them with their own bodies,” Siddall says. “They’ll lay the eggs, cover them with their bodies, and fan the eggs to prevent fungus or bacteria from getting on them, and then when the eggs hatch, they will attach to the parent. They’re not feeding on the parent, just hanging on, and then when the parent leech goes to its next blood meal it’s carrying its offspring to its next blood meal. That’s pretty profound parental care, especially for invertebrates.”

10. You might be the next to discover a new leech species. 

Despite living side-by-side with leeches for thousands of years, we’ve still got a lot to learn about them. Scientists are aware of about 700 different species, but they know there are many more out there. “I’ll tell you what I wish for,” Siddall says. “If you ever get fed on by a leech, rather than tearing off and burning it and throwing it in the trash, maybe observe it and see if you can see any color patterns. Understand that there’s a real possibility that it could be a new species. So watch them, let them finish. They’re not gonna take much blood. And who knows? It could be scientifically useful.”

22 Weird Jobs From 100 Years Ago

Metal Floss via YouTube
Metal Floss via YouTube

Before everyone started working in tech, people actually had their choice of eclectic and strange vocations that put food on their old-timey tables. Discover what lamplighters, lectores, and knocker-uppers did back in the day as Mental Floss editor-in-chief Erin McCarthy runs down 22 Weird Old Jobs from 100 Years Ago.

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