How do we know if something is true?

It seems like a simple enough question. We know something is true if it is in accordance with measurable reality. But just five hundred years ago, this seemingly self-evident premise was not common thinking.

Instead, for much of recorded history, truth was rooted in scholasticism. We knew something was true because great thinkers and authorities said it was true. At the insistence of powerful institutions like the Catholic Church, dogma was defended as the ultimate source of wisdom.

But by the 1500s, this mode of thinking was increasingly being questioned, albeit quietly. Anatomists were discovering that the human body did not function as early physicians described. Astronomers were finding it hard to reconcile their measurements and observations with the notion that the Sun revolves around the Earth. A select few alchemists were starting to wonder if everything really was composed of earth, water, air, fire, and aether.

Then, a man came along that refused to question quietly. When Italian academic Galileo Galilei looked through his homemade telescope and saw mountains on the moon, objects orbiting around Jupiter, and phases of Venus showing the Sun's reflected light -- all sights that weren't in line with what authorities were teaching -- he decided to speak out, regardless of the consequences.

In The Starry Messenger, published in 1610, Galileo shared his initial astronomical discoveries. He included drawings and encouraged readers to gaze up at the sky with their own telescopes. Thirteen years later, in The Assayer, Galileo went even further, directly attacking ancient theories and insisting that it was evidence wrought through experimentation that yielded truth, not authoritarian assertion. Finally, in 1632, Galileo penned the treatise that would land him under house arrest and brand him a heretic. In Dialogue Concerning the Two Chief World Systems, Galileo cleverly constructed a conversation between two fictional philosophers concerning Copernicus' heliocentric model of the Solar System. One philosopher, Salviati, argued convincingly for the sun-centered model, while the other philosopher, Simplicio, stumbled and bumbled while arguing against. At the time, "Simplicio" was commonly taken to mean "simpleton." Simplicio also used many of the same arguments the Pope employed against heliocentrism. At the the time, the Catholic Church was not opposed to researching the topic, but they did have a problem with teaching it. Thus, the Vatican banned the book and imprisoned Galileo.

By stubbornly refusing to be silent, Galileo irrevocably altered the very definition of truth. Scientists today forge breakthroughs in all sorts of fields, but their successes can ultimately be attributed to Galileo's breakthrough in thought. In her recent book, Galileo's Middle Finger, historian of science Alice Dreger paid tribute to the legendary astronomer.

"Galileo actively argued for a bold new way of knowing, openly insisting that what mattered was not what the authorities... said was true but what anyone with the right tools could show was true. As no one before him had, he made the case for modern science -- for finding truth together through the quest for facts."

Primary Source: Galileo's Middle Finger: Heretics, Activists, and One Scholar's Search for Justice, by Alice Dreger. 2015. Penguin Books.