Volume and Momentum

 

It is indeed a matter of great difficulty to discover and effectually to distinguish the true motions of particular bodies from the apparent, because the parts of that immovable space in which these motions are performed do by no means come under the observations of our senses.-Newton

in this passage from the Scholium of the Philosophiae Naturalis Principia Mathematica (1687), Isaac Newton is addressing the core philosophical challenge of his framework: how to distinguish true (absolute) motion from apparent (relative) motion when absolute space itself cannot be seen, touched, or directly measured.

It is indeed a matter of great difficulty to discover and effectually to distinguish the true motions of particular bodies from the apparent, because the parts of that immovable space in which these motions are performed do by no means come under the observations of our senses.-Newton

If everything in a system accelerates together uniformly, relative positions don't change at all, masking the "true" physical state.

Newton argued that true motion reveals itself through forces and physical effects—specifically inertia, acceleration, and rotation.

To prove that true motion exists even if space is invisible, Newton offered his famous Bucket Thought Experiment:

Bucket suspended by a twisted cord: The bucket is filled with water and released.

Initial rotation: At first, the bucket spins, but the water inside remains stationary. The water surface remains flat, despite high relative motion between the bucket and the water.

Full rotation: Eventually, friction catches up, and the water spins at the same speed as the bucket. Relative motion between the bucket and the water drops to zero—yet the water surface now curves up the sides into a concave shape due to centrifugal force.

Newton argued that this surface curvature (a physical effect) proves the water is in true, absolute rotation against the invisible framework of absolute space, rather than merely rotating relative to the bucket itself.

  

  Historical Significance

This passage laid the foundation for classical dynamics, asserting that physics requires an absolute inertial frame to make sense.It remained the dominant framework until Ernst Mach questioned whether inertial forces were generated by motion relative to the distant total mass of the universe (Mach's Principle), which ultimately helped inspire Albert Einstein to replace absolute space with dynamic spacetime in Special and General Relativity.

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