Displacement and velocity are relative, so an object travelling all on its own, with no sensors to compare itself to other objects, will not be able to tell the difference between standing still and moving. This is contrasted with acceleration, which is claimed can be detected intrinsically, e.g. by an accelerometer.

But such an acceleration is detected because when an object is accelerated, the acceleration is usually not uniform. Either the front is pulled or the back is pushed, which creates tension or compression forces, which can be detected internally. But imagine an object is accelerated in a magnetic field in such a way that every one of its particles is accelerated with exactly the same magnitude and direction (probably not perfectly achievable in practice, but we're talking purely theoretically here.) Would such an object (again, in the absence of external sensors) actually be able to tell it's accelerating?