Thermodynamics is a language developed to describe physical observations. Just as mathematics is not any language, but is logic added to language, so thermodynamics is stability added to language. Both languages are constructed to avoid discussing things which do not exist. The nature of this mistake is different in the two cases. Mathematics precludes discussion of objects having contradictory properties, which cannot exist at all. Thermodynamics precludes discussion of objects which do not last long enough in the time frame of interest. In the narrowest sense, it limits itself to those which would persist forever, if the circumstances in which they were created did not change. Such objects are said to be stable under those circumstances.