It is not possible to verify the universality of gravitational forces directly; you would have to study all pairs of bodies in the entire universe, and find a way to remove the influence of the other bodies. But that’s now how science works. Instead, it employs a mixture of inference and observations. Universality is a hypothesis, capable of being falsified every time it is applied. Every time it survives falsification – a fancy way to say it gives good results – the justification for using it becomes a little stronger.
- From In Pursuit of the Unknown, by Ian Stewart