At the time it was proposed, it was thought that a thermal reservoir on its own could never do useful work. This is always what's observed in practice and laws

At the time it was proposed, it was thought that a thermal reservoir on its own could never do useful work. This is always what's observed in practice and laws of heat were based around it e.g. U = Q - W. In order to derive those heat equations from the second law, we now know it requires an assumption: that you (or any entropy-containing component not modeled in the system) cannot have specific knowledge of the microstate of the system, only its macroscopic properties. For a long time it seemed absurd that such an assumption would necessary at all, for these seemingly universal laws, and there was no clear way to thermodynamically model the knowledge of the actor inside this system such that you wouldn't need such an assumption.