If I have two spheres of the same radius, they can still have different temperatures. However: What if the colder sphere is closer to the ideal of being a blackbody than the hotter sphere? http://en.wikipedia.org/wiki/Emissivity In an open system without internal energy generation, both spheres would get colder. However, the colder sphere would lose energy faster than the hotter sphere. In other words, the temperature difference between the two spheres would increase. However, if enough of the energy of one sphere were to be lost to the other sphere, and vice versa, then it would mean that the hotter sphere would receive more energy than it lost, while the colder sphere would lose more energy than it gained. What is obvious is a net transfer of energy. What is not obvious is what exactly that would do to the hotter sphere. Would it get hotter? Or would contribute to a phase change in the hotter sphere?