I think both, I'm not a physicist, so I may get this wrong.
If it wasn't true, if there was a difference in some physical law due to your motion, you would be able to detect the One True Frame of the Universe. Now here's where I get a little fuzzy, I think it was up for grabs, and then they tried (Michelson–Morley experiment https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_exper... ) and couldn't do it. (See also Mach https://en.wikipedia.org/wiki/Ernst_Mach#Philosophy_of_scien... )
If you're in a spaceship with no windows, out in space, and you are in free-fall (not accelerating), there's no experiment you can do that will tell you whether you are moving or not.
If your spaceship has a window, and you look out and see another spaceship passing by (but nothing else) you can't tell whether you're still and it's moving, or you're moving and it's still, or you're both moving. You can detect the relative velocity between you and the other ship, but not the absolute velocity (because there isn't one.)
Intuitively we are used to a "absolute" reference frame provided by the bulk of the Earth but that's just a (highly) local context.
In space (which is to say, in our Universe) there's no absolute reference frame, so you literally cannot have an absolute velocity, and obviously you can't have a physical law described by an equation that includes a term that doesn't exist, n'est-ce pas?
Cheers!