In Newton's $3$rd law of motion, it states that

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> All forces between two objects exist in equal magnitude and opposite direction
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How did Newton knew exactly that the "reaction" force will be equal in magnitude to the "action" force? What would happen if the "reaction" force ,say, half the "action" force?

For example, if I push a small rock with my fingers and the rock moves, the rock is also pushing on my fingers with the same amount of force (and opposite the direction) but because of the difference in mass, the rock is the one moving.

But how do we know for sure that the rock is also pushing on my fingers with the **same amount** of force and not half of it (or a quarter)? What would happen if the rock is pushing on my fingers half of the force I'm pushing on it?