I want to know the answer to the following question:
What asserts that energy is always conserved and that heat flow is a form of energy transfer.
All the background I'm provided with is that "For homogeneous systems, with a well-defined temperature and pressure, a commonly used corollary of the first law is that, for a system subject only to pressure forces and heat transfer (e.g., a cylinder-full of gas) without chemical changes, the differential change in the internal energy of the system (with a gain in energy signified by a positive quantity) is given as". Is that enough to answer the question?
Answer: no