Someone in an inertial frame cannot tell whether they are moving, or whether they are stationary and things around them are moving. You've probably experienced this feeling when you are sitting on a train waiting for it to leave the station, and another train pulls in beside you. Often, this creates the illusion that you have started moving, and that the other train is stationary. This effect is the source of the quote commonly attributed to Einstein (which he probably never said):
As far as he was concerned, it didn't matter if the train was moving and the earth was stationary, or if the earth was moving and the train was stationary. The net result would be identical, and someone sitting on the train or outside it wouldn't be able to discern the difference!
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