Credible theories must include rules to measure their own terms

Newton's breakthrough wasn't just proposing that gravity follows an inverse-square law. It was providing exact procedures for measuring gravitational force from observable phenomena: planetary orbits, pendulum swings, projectile arcs. "Force" was a theoretical concept. Newton gave it measurement rules.

This is what made Newtonian mechanics testable in a way earlier force concepts weren't. A theory that names its quantities without specifying how to measure them offers no grip. You can't find out if it's wrong. And a theory you can't be wrong about does little work.

The same logic applies to any scale. A thermometer is a physical device, but behind it are theories about thermal expansion, corrections for altitude, rules for calibration. The measurement is never just the reading off the instrument. It's the whole apparatus of ideas that says what the reading means and when to trust it. Building that apparatus well takes serious effort, often generations.

Source claim: A theory gains scientific credibility not just by naming theoretical quantities, but by providing explicit rules for measuring those quantities from observable phenomena.