In classical physics, time became a measurable and calculable quantity. Technical advances in mechanical engineering made possible the construction of increasingly precise chronometers and clocks. With the aid of modern mathematics, time could be measured with arbitrary precision. In the formalism of classical mechanics, time is merely a coordinate in the equations of motion. These equations remain valid under certain transformations—for example, those in which the direction of time is reversed. But the invariance of time as a constant measurable quantity—the notion that time is an absolute, independent, unvarying entity—was fundamental in classical mechanics. Only with the advent of relativity and then quantum theory were these notions of an invariant and independent entity called into question. They have also been considered by modern epistemology, where time is investigated as a form of consciousness.