Newton’s Philosophiæ Naturalis Principia Mathematica (1687) introduced absolute time: “true and mathematical time, of itself, and from its own nature, flows equably without relation to anything external.” In Newtonian dynamics, the equations of motion (e.g., ( F = m \frac{d^2x}{dt^2} )) are time-symmetric . If you reverse ( t ) to ( -t ), the equations remain valid. A film of two colliding elastic balls played backward shows equally valid physics. Thus, classical mechanics contains no inherent arrow of time; the distinction between past and future is purely a boundary condition imposed on the universe, not a law.
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Science does not always progress smoothly. As described by Thomas Kuhn, it goes through periods of "normal science" punctuated by "paradigm shifts"—radical changes in worldview (e.g., moving from Newtonian physics to Quantum Mechanics). Thus, classical mechanics contains no inherent arrow of