Mechanics, Volume I

...all bodies, insofar as they are such, possess a force or faculty of remaining perpetually in their state, which is nothing other than the force of inertia original: vis inertiae itself. Indeed, the name "force" is attributed less aptly to the cause of this conservation, because it is not of the same kind original: homogenea as other forces properly so called, such as the force of gravity original: vis gravitatis, nor can it be compared with them; an error in which many, and especially the Metaphysicians Euler is likely critiquing those who treated inertia as an active, internal push rather than a passive property, are accustomed to be engaged, deceived by the ambiguity of the word. Since, therefore, every body by its nature perseveres in the same state, whether of rest or of motion, it must be attributed to external forces if a body does not follow this law, but instead proceeds either with unequal motion or along a curved line.

These types of external forces are indeed powers potentiae: external forces or agencies that change a body's state, whose equilibrium and comparison must be treated in Statics The branch of mechanics dealing with bodies at rest or in equilibrium; when these act upon a body, they disturb its state, either by moving it, accelerating it, slowing it down, or changing its direction.

In the second Chapter, therefore, I investigate what kind of effect each power ought to exert on a free point, whether it is at rest or in motion. From this are established the true principles of Mechanics, from which whatever pertains to the alteration of motion must be explained; these principles, although they have hitherto been too lightly confirmed, I have demonstrated in such a way that they are understood to be not only certain but also necessarily true. Having set forth the principles, from which