[Footnote 56: I first drew attention to this fact in my treatise _Ueber die Erhaltung der Arbeit_, Prague, 1872. Before this, Zeuner had pointed out the analogy between mechanical and thermal energy. I have given a more extensive development of this idea in a communication to the _Sitzungsberichte der Wiener_ _Akademie_, December, 1892, entitled _Geschichte und Kritik des Carnot'schen Wärmegesetzes_. Compare also the works of Popper (1884), Helm (1887), Wronsky (1888), and Ostwald (1892).] [Footnote 57: Sir William Thomson first consciously and intentionally introduced (1848, 1851) a _mechanical_ measure of temperature similar to the electric measure of potential.] [Footnote 58: Compare my _Analysis of the Sensations_, Jena, 1886: English translation, Chicago, 1897.] [Footnote 59: A better terminology appears highly desirable in the place of the usual misleading one. Sir William Thomson (1852) appears to have felt this need, and it has been clearly expressed by F. Wald (1889). We should call the work which corresponds to a vanished quantity of heat its mechanical substitution-value; while that work which can be _actually_ performed in the passage of a thermal condition _A_ to a condition _B_, alone deserves the name of the _energy-value_ of this change of condition. In this way the _arbitrary_ substantial conception of the processes would be preserved and misapprehensions forestalled.] THE ECONOMICAL NATURE OF PHYSICAL INQUIRY.[60] When the human mind, with its limited powers, attempts to mirror in itself the rich life of the world, of which it is itself only a small part, and which it can never hope to exhaust, it has every reason for proceeding economically. Hence that tendency, expressed in the philosophy of all times, to compass by a few organic thoughts the fundamental features of reality. "Life understands not death, nor death life." So spake an old Chinese philosopher. Yet in his unceasing desire to diminish the boundaries of the incomprehensible, man has always been engaged in attempts to understand death by life and life by death.