Descartes believes all true motions are relative. An object’s motion is always arbitrary because it can never hold as absolute property. The following example fully illustrates his concept of true motion and “vicinity”. Imagine a universe that has only two spaceships, nothing else, that are moving at the same speed in the same direction. When a person on spaceship A looks at spaceship B, it won’t seem like the B is moving. When this person adopts the bird’s eye view and is at rest, both spaceships are moving away from him/her. Although the spaceships are in the same motions, they can be seen as moving and not moving, depending on the point of view of the person. To Descartes, this inconsistency implies true motions are relational. Defining a motion relationship requires comparing at least two participants. Regarding a motion-based relationship, one needs at least two objects to form an arbitrary relationship to make a comparison. Either object can be referred to as “vicinity” because “vicinity” only identifies the other participant being compared and described in this relationship. In the first scenario, spaceship A’s motion is described by spaceship B, which is the “vicinity” in the relationship, or it can be the other way around that makes A the “vicinity” when one describes the motion of B. In the second scenario, both spaceships are described by the person’s motion, above at rest. The person is the “vicinity” that explains both spaceships; and again, the reverse also applies. The “vicinity” simply is the other object that serves as a benchmark or a point of reference to describe the first object’s motion. It’s important to note that Descartes thinks an object’s motion is dependent on every other object. All “vicinities” are equally valid benchmarks, so the definition of this object’s motion is entirely arbitrary. During comparisons, its motion varies from “vicinity” to “vicinity.”
Since all motions are relative and thus arbitrary to Descartes, he concludes no one universal benchmark, or universal “vicinity,” can exist to consistently account for all objects’ motions. His idea of true motion, in other words, is that every object’s motion is determined by every other object in the universe. Every object is a metric system for explaining the motion of every other object; this idea of true motion in his theoretical universe allows no standard point of reference. Because Descartes thinks true motion is a relationship between two objects, one single object cannot form a relationship in an empty world without a universal “vicinity” to act as the second participant. In short, this object’s motion simply doesn’t exist without any point of reference to form a relationship with.
Newton, on the other hand, accepts that motions can be defined as relationships between among objects or groups of objects but disagrees with Descartes on that a universal benchmark for motion doesn’t exist. Newton’s faith in the universal benchmark is what substantially differentiates his theory from that of Descartes. He uses the bucket experiment to present a manifestation of this universal benchmark in a perceptible way. A rope is tied to an object and suspends it to a ceiling. The experimenter twists the rope as much as possible, waits for the water to settle, and lets the rope go. The rope will cause the bucket to rotate. The water’s surface will remain stationary and flat temporarily and start gradually forming a vortex. When the bucket’s rotation slows down as the rope loosens, the vortex will surprisingly circulate even faster than the bucket. The inconsistency is that while the water spins in relation to the bucket, the surface doesn’t. The bucket is clearly not a reliable “vicinity” benchmark because it doesn’t fully interpret the water’s motion. However, in Descartes’ world, two vicinities will always be able to adequately justify each other’s motion perfectly because motions are completely arbitrary. Newton has created a phenomenon that Descartes’ theory fails to account for. Something besides relative space should be present to gauge the motion of the water because the only “vicinity” doesn’t justify the motion of the water’s surface.
To explain the water surface, one must find another one to supplement it to solve the inconsistency. Newton theorizes that a universal “vicinity,” or benchmark, is elsewhere that cannot be perceived but constantly present and influential—absolute space. This idea results in a significantly different physical universe than Descartes’. Because this absolute space constantly functions as a point of reference, any object is always in a relationship of motion with it, so motion can exist in a Newton’s single-object world.
Descartes can respond by complicating his theory and allowing hierarchies of points of reference to depict motion, meaning that one “vicinity” can influence an object’s motion better than the other. Even though the bucket is not the only “vicinity” in Newton’s experiment, it’s possible that another extremely powerful but distant “vicinity” similar to the nature of absolute space is affecting the surface of the water. There can be more than one “vicinity” that exerts the benchmark effect on the motion of an object, and this powerful object can simultaneously serve as the second point of reference with the bucket. This “vicinity,” in contrary to absolute space, is not absolute; its benchmark effects may only apply to the solar system or the galaxy together, so it’s not omnipresent like absolute space.
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