The Trial of Galileo
Galileo Galilei, one of the greatest minds in history, looked through this newfangled thing called a telescope and realized that the Earth couldn’t be at the center of the universe. He wrote this down, expanding the boundaries of human knowledge.
The Catholic church recognized that this clearly contradicted religious belief at the time and declared Galileo to be a heretic. He told the inquisition that they could look through his telescope, they would see the movement of the moons of Jupiter, and being intelligent people could reach no other conclusion than his own.
Most of them refused even to look in the telescope. Those who did agreed with him, but testified for the prosecution anyway because they were priests after all and if the church insisted that they prosecute a genius in the name of enforced ignorance then by golly, that’s what they’d do.
Galileo was condemned and forced to recant (which he did while muttering under his breath “but still, it moves”, referring to the Earth).
Racism is a genuine example of ignorance. To be honest, I can understand how at one relatively brief time in human history, some people looked around and said “hey, we Europeans are dominating the planet, we must be better than non-Europeans”. There’s a hypothesis there, and while it’s not very PC to say it, for many years that made sense to a lot of people.
But for many years it also made sense to a lot of people that the sun went around the earth. Racial supremacy is false as well. In reality, race is not really a thing and we now know that there are far more genetic differences between two people of the same “race” than there are across groups that have traditionally been called “races”. Race is no more a guide to human typology than is being left or right handed, or eye color or hair color. This really has been studied and it’s just as much a fallacy as my other examples.
Therefore, entire movements built around either the idea that your racial type is superior or that some other race is inferior or even really fundamentally different in any substantive way is just as false and disproved as any other stupid idea and acting on that sentiment is woefully ignorant.
Before Louis Pasteur popularized the idea of germ theory, people, both scientists and laymen alike, had very different ideas about how disease was spread and what caused it. The 3 most popular ideas of the time were:
The miasma theory was popular for a very long time as a way to explain how diseases were spread. The thought was that the sickness passed to people through malodorous smells
The Four Humors
The four humors theory stated that the body was made up of four humors, or substances. These were yellow bile, black bile, blood, and phlegm. When a person became ill, the theory surmised that it was because these four humors were out of balance.
This theory suggested that sickness was spread through touch, and that if a person did not come into bodily contact with the ill that they would remain healthy.