First-principles is one of my favorite thinking styles.
Consists of looking at whatever understanding you have and determining what are the underlying facts and principles that support the understanding. It is a process of questioning your understandings and the associated assumptions.
This is harder than people realize.
Hypothetically, most understandings are opinions. A theory is a special type of opinion. Good theories, that we somethings take as fact, are based on a large amount of observation and are built on facts.
It is a set of theories that say the sun will come up tomorrow. Therefore, the sun coming up tomorrow is an opinion/theory. These theories are well-founded on fact/ experimental observation so we can be extremely confident in the theory and take it as fact.
Too often, people take opinions to be facts and don’t take the time to determine how solid an opinion is.
Opinions are beliefs, theories, opinions, ideas.
The process is to find and understand the supporting facts and supporting opinions for an opinion. Supporting opinions must be examined to find the supporting facts and opinions for those until you have the lowest level possible.
The next step is to examine how you have decided to put the facts and opinions together to form your new opinion and to see how the facts support your opinion and how the facts contradict your opinion (or point to a different opinion)