Galileo Galilei is quoted as saying that "where the senses fail us, reason must step in."

Then again, he also made astrological charts for rich people.

Throughout history, scientific geniuses from Galileo to Newton have often believed in completely fantastical things — from astrology to alchemy to straight-up magic. Yet some of their bizarre ideas seemed completely valid to them at the time, and often for very good reasons.

We have the benefit of hindsight today, which gives us an unfair advantage over these geniuses. But that doesn't make their beliefs seem any less weird. Let's take a look:

1) Galileo believed astrology changed everything

Today, Galileo (1564–1642) is held up as a paragon of rationality. He advocated heliocentrism — the idea that the sun, not the Earth, was at the center of the solar system — fought an anti-heliocentric church at great risk, and greatly advanced astronomy throughout Europe.

He also was something like a fortune teller.

Galileo didn't just believe in astrology: he practiced it, conducted it for wealthy clients, and taught it to medical students. If students at the University of Padua had taken MCAT, Galileo would have included a question about whether a Leo should date a Gemini.

Galileo wasn't alone in keeping up on his signs. His contemporary Johannes Kepler conducted his own astrological studies, though more reluctantly (he called people who believed in astrology "fatheads").

2) Isaac Newton thought alchemy was the future

John Maynard Keynes called Isaac Newton (1642–1726) "the last of the magicians" with good reason. Newton spent half his life obsessed with alchemy, the transformative magic most frequently associated with turning different metals into gold. To make things even more complicated, in 1696, Newton became warden of the Mint, and he became master of the Mint in 1700. The Royal Mint, of course, makes the coins for the entire United Kingdom (which was formed in 1707, 7 years into Newton's appointment). To be clear: an alchemist was the person in charge of making all the money.

Newton wasn't the only respected mind who had visions of diving into gold coins. Robert Boyle is considered the father of chemistry, but he dabbled in alchemy, as well. In fact, he was so committed to the alchemical cause that he fought to make alchemy legal, since Henry IV had banned it (because alchemy wasn't good for the monetary supply). Needless to say, the repeal wasn't necessary.

The philosopher's stone Newton chased after wasn't only able to "cure" metals that weren't gold — it also had medical powers that fascinated Newton and his peers. Unfortunately, today you can only find the philosopher's stone in the British subtitle of the first Harry Potter book.

3) Tycho Brahe made everyone believe he was a sorcerer

Tycho Brahe (1546–1601) created his own model of the universe, and though he didn't get things quite right, he helped advance astronomy and catalogued more than 1,000 stars. He also convinced everyone he was a sorcerer.

He did so from the unique perch of his private sorcerer's island, Hveen (today known in English as Ven). Fantastically wealthy, Brahe built multiple observatories there, had a squad of astronomical assistants, and used tiny automota (robots) to convince the locals he had magic powers. It didn't hurt that he partied hard, had his nose partly sliced off in a duel, and got his pet moose drunk at parties.

But he didn't just hoodwink the public into believing he was magical — he believed it too. He publicly lectured against anyone who believed astrology was fake, and he also believed alchemy was the future for mystical discoveries. Brahe even became so synonymous with magic that an entire calendar of magical days was made in his honor (and his name was slapped on to give it magical credibility).

4) Carl Linnaeus classified magical animals like the hydra and believed in mermaids

Carl Linnaeus (1707–1778) imposed taxonomical order on animal and plant life. In his era, scientists were discovering all sorts of new species at a rapid clip (Linnaeus himself thought pelicans might be a myth). That rapid pace of discovery led Linnaeus to believe, perhaps reasonably enough, that humans would soon find a host of mythological animals.

Linnaeus devoted a whole section of his landmark Systema Naturae to these strange beasts. It was called Animalia Paradoxa and included:

the hydra

the satyrus (a monkey-like man, similar to Pan in Greek mythology)

the phoenix (the bird that rose from the ashes)

Did Linnaeus believe in these animals? It's hard to know, and some of Linnaeus's defenders say he only included the animals to point out how absurd they were. In the 1730s, he became famous for debunking a hydra in Hamburg. However, we can reasonably claim that Linnaeus believed he'd found a troglodyte, was pretty confident he'd seen a unicorn horn, and was very excited at the chance to find a mermaid.

Whatever the motivation, Linnaeus wasn't alone in believing in bizarre, vaguely magical animals. Gottfried Leibniz managed to help invent calculus, yet he still wanted to fill a museum with weird (and imaginary) animals like the myrmecoleon (some sort of ant-lion).

5) Paracelsus loved natural magic and himself

Paracelsus (1493–1541) did a lot when he was alive, including basically inventing toxicology and naming zinc. But when he wasn't revolutionizing scientific methods and naming metals, he was a big fan of magical things.

Born as Philippus Aureolus Theophrastus Bombastus von Hohenheim, he renamed himself Paracelsus, both because it was shorter and because it literally meant he was "better than Celsus," a first-century Roman medical researcher (in Paracelsus's defense, he may have been renamed by his biggest fans). Paracelsus wrote that from an early age the "transmutation of metals" was his obsession, and he pursued it with vigor as an adult.

When he wasn't traveling the world performing surgeries, he tried to utilize "natural magic" to help patients. He was quoted as saying "magic is a great secret wisdom," and while his understanding of natural magic occasionally lent itself to scientific inquiry, he also believed that "the soul strongly desires sulphur." As the scientist on this list closest in time to Aristotle, it makes sense that Paracelsus would indulge in magic and the occult.

In his defense, that belief in magic was grounded in a commitment to inquiry: Paracelsus thought magic was just science that wasn't understood yet. In a way, that unites all the scientists on this list, who pursued new knowledge even when it meant looking in some very unusual places.