Galileo had the right idea

Galileo Galilei (1564-1642) was far ahead of his time when he formulated his own 'principle of equivalence'. He wrote: 'Shut yourself up with some friend in the largest room below decks of some large ship, and there procure gnats, flies, and such other small winged creatures. Also get a great tub full of water and within it put certain fishes; let also a certain bottle be hung up, which drop by drop lets forth its water into another narrow-necked bottle placed directly under it. Having observed all these particulars as long as the vessel stands still, how the small winged animals fly with like velocity towards all parts of the room, how the fishes swim indifferently towards all sides, and how the distilling drops all fall into the bottle placed make the ship move with what velocity you please, so as the motion is uniform and not fluctuating this way and that____You will not be able to discern the least alteration in all the above-mentioned effects, or gather by any of them whether the ship moves or stands still.' He stopped short of questioning what exactly was meant by 'standing still'!

Galileo's Gedanken experiment

Galileo used a thought experiment to illustrate his claim that the earth is moving around the sun, and that this fact would not affect experiments involving falling objects. He claimed that the water droplets will continue to fall directly into the bottle below, no matter whether the ship is standing still or moving with a steady velocity in any direction.

In current terminology we say that the experiment will work identically in all inertial (Galilean) frames of reference.

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