(Berlin, Haude et Spener, 1750). 4to. No wrappers, as issued in ""Mémoires de l'Academie Royale des Sciences et Belles-Lettres"", 1748, tome IV, Titlepage to the section. a. pp. 324-333.
First appearance of this importent paper in which Euler defends Newton's conceptions of space and time against the thesis that space and time are ideal, and motions relative. He outlays his views on the relation between Metaphysics and Mechanics. The truths of mechanics are ""so indubitably constant"" that they must be founded in the natures of bodies. Metaphysics is the study of the nature of bodies, therefore the laws of Mechanics constrain Metaphysical theories. In fact, any Metaphysical idea or conclusion corresponding to a Mechanical one must agree in all its implications with Mechanics. This applies in particular to space and time. Real, absolute, space and time are assumed by the laws of Mechanics. Therefore, Metaphysical arguments for the unreality of space and time must be unfounded and ""hide some parlogism"".