How do you know?
I am quite well read, and have studied and researched many subjects. It is quite rare that I enter into a conversation or debate without bringing long and detailed arguments and opinions into play.
It is therefore inevitable that I am asked how I know what I know. In many cases, arguments have been posed against me based on the fact that my information has come from other sources, which then amounts to hearsay.
In the area of religion, for instance. I might argue that the Christian believes in the words of people who wrote them centuries ago. I say that this is hearsay and such material is invalid. The opposition argues that all the information that I hold to be valid was also penned by men. The fact that I have never personally gone to the center of our milky way galaxy and observed the massive black hole invalidates the proposition that a massive black hold resides there.
The thing is, however, that I have weighed the evidence that supports all contentions and chosen to acknowledge the most reasonable view as the correct one. To my mind, there is no centralised authority to which I must defer to. If someone says something and is able to support their stance with solid evidence, then it is safe to say that they are correct.
Thus, it is a matter of asking the right questions and only accepting the most reasonable answer.
By contrast, many people tend to only ask questions and accept answers that support their personal bias. They don't want real answers, but are looking for something simplistic that will make them feel like they are more informed and still uphold their previous assumptions.
Aristotle did an experiment on living slaves in which he opened them up to see how they functioned. He examined the veins and arteries that flowed through their bodies. He saw oxygenated blood flowing and wrote about it. Then he looked to the blue veins full of unaerated blood. He proclaimed in his writings that they carried air, and were the physical seat of the soul. When asked why blood flowed from them when they were opened, he had to be very inventive in order to support his previous proposition.
Aristotle began with the premise that there was a soul, and a physical seat within the body where that soul resided. He then went to great lengths to find that seat.
Moreover, once he made a proposition, he would go to great lengths to support it even in light of contradictory evidence. Nobody wants to be proven wrong, after all. Even more importantly, if he had accepted that he had been wrong in any assessment, it would have proven his own fallibility and provided a weakness that could have been exploited.
I know what I know because I have researched and studied material from those who are willing to acknowledge when they are wrong. Likewise, I am perfectly willing to acknowledge and admit it when a proposition that I uphold is proven to be in error.
How do I know that what I know is correct? I don't, really. I only know that the observable universe supports those things that I accept as correct. I don't see any evidence for invisible pink unicorns moving around and making candy, for instance, but I do find plenty of answers to questions of life in evolutionary theory. That would be why I accept evolutionary theory as factual and spirituality as fanciful. I know the questions to ask, and can therefore make an attempt to find real answers.
I am become death, destroyer of worlds