Internal energy U is a state function, meaning its change can be inferred just by comparing two different states of the system (independently of its transition path), which we can therefore indicate with U 1 and U 2. Since we can go from state U 1 to state U 2 either by providing heat Q = U 2 − U 1 or work W = U 2 − U 1, such a change of state does not uniquely identify the amount of work W done to the system or heat Q transferred, but only the change in internal energy Δ U.