The first law of thermodynamics is based on the principle of

The first law of thermodynamics states that the heat supplied to a system is equal to the sum of the change in its internal energy and the work done by the system.

Mathematically, $\Delta Q = \Delta U + \Delta W$.

This principle implies that energy can neither be created nor destroyed, but only converted from one form to another. Therefore, it is based on the law of conservation of energy.