Everything you know is in your brain. For what you don't, you can ask the web. However, as you can imagine, in the past there were no computers and no web at all. Think about people living 3, 500 years ago, in places where pyramids towered above them. The lack of explanations about how things worked inspired the ancients to make discoveries and feed their curiosity.
But do not think they knew nothing: in fact, we still wonder how the Egyptians could build the pyramids using their own, rather rudimentary equipment. They wrote down their knowledge on books (called papyrus scrolls) so that later people could benefit from their skills.
Living safer and healthier was a top priority for the ancients: when they hurt their heads in battles,
they used to heal their wounds using the cures, some of which were recorded on a papyrus now known as the Edwin Smith Papyrus. Interestingly, this is the oldest written evidence of the word "brain" we have on Earth, and it dates back to 3, 500 years ago!
However, writing the word "brain" on a papyrus did not solve all the curiosities and questions about it. Around 2, 500 years ago, for example, ancient Greeks wondered whether mind and soul lay in the brain or in the heart.
2, 000 years ago, Roman physician Galen of Pergamon was sure he had solved the question: it was the brain, not the heart, that was the center of mental life. However, Galen had to convince the people around him that what he had discovered was true. It was not easy, mostly because people held their earlier beliefs. Every neuroscientist, just like Galen did, has to carefully observe reality, focus on something that is really interesting, think about how it can work, perform experiments, and discuss the data they get. This way, they can obtain convincing evidence to be shared with others.