克劳修斯在1865年发表的著名论文中[Clausius R. Mechanical Theory of Heat [M], London: John van Voorst, 1867], 对他自己的热力学研究作了一次系统的总结: 引入了熵函数, 得到了克劳修斯不等式 dS >= dQ/T, 以及适用于孤立(或绝热)体系或熵增原理. 在克劳修斯的同一篇论文中, 克劳修斯还说: (p. 364)
第二基础原理, 在我所给出的形式中, 断定所有在自然界中的转变可以按一定的方向, 就是我已经假定是正的方向, 而不需要补偿地由它们自己进行; 但是对相反的方向, 就是负的方向, 它们就只可能在同时发生的正转变的补偿下进行. (The second fundamental theorem, in the form which I have given to it, asserts that all transformations occurring in nature may take place in a certain direction, which I have assumed as positive, by themselves, that is, without compensation; but that in the opposite, and consequently negative direction, they can only take place in such a manner as to be compensated by simultaneous occurring positive transformations. )