A first principle is a foundational proposition or assumption that stands alone. First principles can't be deduced from any other proposition or assumption. The inverse is reasoning by analogy. An example of first principles are the rules of soccer. These rules describe what you can or cannot do.
Farnam Street describes the distinction between reasoning by first princples or by anologies as:
Another way to think about this distinction comes from another friend, Tim Urban. He says[3] it’s like the difference between the cook and the chef. While these terms are often used interchangeably, there is an important nuance. The chef is a trailblazer, the person who invents recipes. He knows the raw ingredients and how to combine them. The cook, who reasons by analogy, uses a recipe. He creates something, perhaps with slight variations, that’s already been created. The difference between reasoning by first principles and reasoning by analogy is like the difference between being a chef and being a cook. If the cook lost the recipe, he’d be screwed. The chef, on the other hand, understands the flavor profiles and combinations at such a fundamental level that he doesn’t even use a recipe. He has real knowledge as opposed to know-how.
Source: First Principles: The Building Blocks of True Knowledge
To establish First Principles several techniques exists. Socratic questioning is an example of such an technique.
This is a disciplined questioning process. One that can be used to establish truths, reveal underlying assumptions, and separate knowledge from ignorance. It is meant to draw out first principles in a systematic manner. Generally it follows this process: