We understand both motion and rest are relative. But me and my friends always discussing about this kind of thought experiment involving a single object (for example, a particle with mass or massless particle) with nothing else existed (no other objects, no force, no gravity, no quantum field, etc.). Because motion is relative, then someone can say the lone object is at rest. It is motionless, and someone can also say the natural state for this lone object is to be at rest. But for me, in my opinion this kind of thought experiment is actually impossible in reality and this thought experiment is purely imaginative. It is impossible and irrational because both motion and rest (motionless) are relative so this lone object with nothing else existed is actually neither moving nor not moving thus a single object universe is purely imaginative. What do you think?
You're right, motion is only a relative concept and without another reference body (or what we physicists would call a "reference frame") the notion of motion cannot really be defined. A fundamental but often implicit assumption in physics is that measurement can be made and reproduced. Only with measurement then we can unambiguously and meaningfully describe our physical world and talk about physics at all. But to make meaningful measurements, you unavoidably need to bring in a measurement device hence a reference body to your universe. So your question is really more a philosophical one than a physical one.