Mach’s foundational 1883 work, The Science of Mechanics, attacked the 19th century ‘scientific’ establishment's unprovable metaphysics. Mach wrote, "No one is competent to predicate things about absolute space and absolute motion; they are pure things of thought, pure mental constructs, that cannot be produced in experience."
Mach is saying that Newtonianism is philosophy not ‘science’. It is entirely unproven and not mechanically evidenced. Mach’s view of the universe is the contrary to that of Saint Newton’s.
Newton: Newton believed space was a real, rigid, physical entity, basically an empty room. In Newton’s water-bucket experiment (link below this section analysing why this is wrong); if we spin a bucket of water hanging from the tree limb, the water climbs up the walls of the bucket due to its rotation ‘relative’ to that absolute, fixed room.
In this experiment the word ‘inertia’ indicates an object's resistance to changing speed relative to space itself. Inertia references an object at rest, or, an object going in the same direction at the same speed (no acceleration, no deceleration). Newton’s maths only work with inertial frames. Compare this to Mach.
Mach: For the Austrian "empty space" is an unprovable abstraction. If God, the ‘Universe’, or Einstein deleted every star, planet, and atom from the universe except for one bucket of water, "rotation" loses all meaning. There is nothing to rotate relative to. Therefore, the water does not climb the sides of the bucket, and you would never feel dizzy if you were spun around.
Einstein agreed with Mach and picked up the theme that knowledge and data interpretation is ‘Relative’ and applied this ‘axiom’ to Saint Isaac of Gravity and his mechanics. It was Messiah Einstein who minted the phrase, "Mach’s Principle" to argue that “local inertia is merely an interaction with the rest of the universe's matter”.
So here we have 2 worldviews: Newton’s absolute space (an empty room) and objects measured against that absolute background; and Mach who maintains that physical relationships must be ‘Relative’ and premised on many variables. More here.