Is the First Law of Thermodynamics an ab initio principle?

Hello, this is an interesting question. Simply put, they are not the same thing, but they do have a certain connection.

You can think of it this way:

The First Law of Thermodynamics, in essence, is the "Law of Conservation of Energy." It tells us a fundamental rule of the universe: energy cannot be created or destroyed; it can only change its "form," transforming from one type to another. For example, when you boil water, electrical energy (or chemical energy from gas) is converted into the internal energy (heat energy) of the water; when you charge your phone, electrical energy is converted into chemical energy in the battery. This is a physical law that has been proven countless times through observation and experimentation, an undeniable truth. It is a "conclusion," a "fact."

First Principles, on the other hand, is not a specific law but a way of thinking or a mental model. Its core idea is: when looking at any problem, you should return to its most basic, core essence (those self-evident "axioms" that cannot be further broken down), and then, starting from these fundamental points, derive conclusions and methods step by step. It encourages you not to think by analogy (e.g., "because everyone else does it this way, so should I"), but to think through everything from scratch yourself.

The most famous example is Elon Musk building rockets. He didn't think, "Rockets have always been expensive," which would be analogical thinking. His first principles thinking was: "What are the most basic materials for building a rocket? Aluminum alloy, titanium, copper, carbon fiber... How much do these things cost on the market?" He started by calculating the cost of the most basic materials and found that if he sourced raw materials himself to build them, the cost could be reduced to 2% of the market price at the time, leading to SpaceX's low-cost rockets.

So, their relationship is:

• The First Law of Thermodynamics is a "fundamental fact," an essence of the physical world.
• First Principles is a "thinking method," and this method requires you to find and utilize "fundamental facts" like the "First Law of Thermodynamics" to analyze and solve problems.

In other words, when an engineer uses "first principles" to design a new engine, they must use the "First Law of Thermodynamics" as one of the cornerstones of their thinking, because no design can violate the conservation of energy. This law is their starting point for reasoning, but the process of "thinking from the most fundamental principles" is what is called "first principles."