I suppose a world whose various parts possess a heat conductivity so perfect that they are constantly in equilibrium of temperature. The inhabitants of this world would have no idea of what we call temperature difference; in their treatises on physics, there would be no chapter devoted to thermometry. Other than that these treaties could be quite complete and they would teach a lot of laws, much simpler even than ours.
Now imagine that this world is slowly cooling down by radiation; the temperature will remain everywhere uniform, but it will diminish with time. I suppose one of its inhabitants falls into lethargy and wakes up after a few centuries; we will admit, since we have already supposed so many things, that he can live in a colder world and that he has preserved the memory of things formerly. He will see that his descendants still do physics treatises, that they continue not to talk about thermometry, but that the laws they teach are very different from those he has known. For example it has been taught that water boils under a pressure of 10 millimeters of mercury, and new physicists will observe that to boil it must be lowered to 5 millimeters. Such a body that he once knew as liquid will only appear in a solid state, and so on. The mutual relations of the various parts of the universe all depend on the temperature and as soon as it changes everything is upset.
Well, do we know if there is not some physical entity, as unknown to us as the temperature was for the inhabitants of this fantasy world? Do we know if this entity does not vary constantly like the temperature of a globe that loses its heat by radiation, and if this variation does not lead to that of all the laws?