First Principles Thinking: Break things down to the most fundamental truths and reason up only from there.
Book: The Great Mental Models Volume 1: General Thinking Concepts
First Principles Thinking is one of the most widely known mental models, especially among the Elon Musk or Charlie Munger fan clubs. The idea is to break down complicated problems into fundamental elements that we know almost for sure are true. We then think through the problems based on those elements without other inherited wisdom or second-handed knowledge.
Reasoning from first principles allows us to think outside history and conventional wisdom and see what is possible. Therefore, it is often used to reverse-engineer complicated problems and develop something no one has built before.
You can find Elon Musk explained this idea and gave the example of how he used First Principle Thinking to question why people said battery pack is always expensive and that's just the way they will always be.
Some ways to build First Principles include Socratic Questioning and The Five Whys. You can also find a detailed explanation of this in the article: First Principles: The Building Blocks of True Knowledge.