This story originally appeared in print in the December 2014 issue of mental_floss magazine. Subscribe to our print edition here, and our iPad edition here.

1. PHLOGISTON

In 1669, alchemist Johann Joachim Becher proposed that fire was caused by an element called phlogiston. Anything you could set alight, he claimed, contained this substance and the only way to put a fire out was to “dephlogisticate it”—that is, burn it to ash. This theory prevailed for nearly 100 years, until Joseph Priestley debunked it by discovering a little thing called oxygen.

2. SPONTANEOUS GENERATION

For ancients like Aristotle, the maggots that wriggled out of rotten meat were living proof that life could bloom at random. As late as the 1600s, Belgian scientist Jan Baptista van Helmont had a recipe for making mice: “Place a dirty shirt or some rags in an open pot or barrel containing a few grains of wheat or some wheat bran, and in twenty-one days, mice will appear.”

3. MIASMA THEORY

Until the mid-19th century, people believed bad smells were a poisonous mist of decomposed matter that caused disease. It wasn’t until the 1860s that germ theory revealed that microorganisms are actually to blame. However, the idea that the world was full of invisible living things seemed so outrageous that for decades doctors debated the necessity of washing their hands.

4. MATERNAL IMPRESSION

Up until the early 1900s, some doctors believed that a pregnant woman’s thoughts and experiences could cause birth defects. For example, if she saw a man lose a hand in a wheat thresher, the shock could cause her child to be born with a stump. Joseph Merrick (a.k.a. “the Elephant Man”) believed his deformities were the result of his mother being frightened by a fairground elephant.

5. EMISSION THEORY

A lot of very smart people—from Plato to Euclid to Ptolemy—believed that we see things because our eyes shoot out beams of light. Of course, this is backward: We see because light enters our eyes. But the idea persisted nevertheless for at least 10 centuries: A 2002 study found that up to 67 percent of college students believed eyes emit rather than receive light.