**Clausius I principle (***CI***)**

*Heat naturally flows from a body at a higher temperature to a body at a lower temperature. Therefore, a direct (not forced by work) process of heat transfer from the body at a lower temperature to the body at a higher temperature is not possible.*

An absolute temperature scale is not needed to determine which reservoir has a higher temperature—even the Celsius scale is sufficient here. The Clausius I principle seems so trivial that one might suppose that it has no far-reaching, less trivial consequences. Nevertheless, it is allegedly proved, that the Clausius I principle is equivalent to the full II law of thermodynamics [12–15]. Criticism of this popular proof could only be found in a 2008 preprint by a scholar Bhattacharyya [16].

The *C I* principle is ascribed to Clausius' publications from 1850, 1851 [17,18] and 1854, 1856 [19,20]. Contemporary work by Xue and Guo suggests that the above assignment is not strict [21]. These authors do not distinguish two Clausius principles (*C I*, *CII*), so it is not surprising that their reference to later Clausius textbooks [22,23] has to do with the second version of the Clausius principle (*CII*). However, in the chapter: *"III. Second Main Principle of the Mechanical Theory of Heat"* and in section: *"5. New Fundamental Principle concerning Heat"* of textbook [23], Clausius wrote: *"...and then enunciate the principle as follows: 'A passage of heat from a colder to a hotter body cannot take place without compensation'. This proposition, laid down as a Fundamental Principle by the author..."* Such a declaration by Clausius from 1879 constitutes his principle in the first version (*C I*).
