Can't understand a statement about motion From the book where I am studying motion, It says

Motion is a combined property of the object under study and the observer. There is no meaning of rest or motion without the viewer.

I know that, for an object, it can be said that 'it is moving' in one frame of reference, and it can be said that 'it is at rest' in another frame of reference, but the sentence I mentioned above seems somewhat confusing. How can a phenomena be a property of two things? Also, how is it that, when there is no one to see, the topic of motion and rest is irrelevant? I don't know exactly what the second sentence is trying to say, provided that my understanding of the second sentence is wrong. I need assistance.
 A: 
Motion is a combined property of the object under study and the observer.There is no meaning of rest or motion without the viewer.

Both of the statements are correct. Let's understand the first statement with an example.
Suppose there are two objects $A$ and $B$ and both are moving in the same direction. The relative velocity of object $A$ with respect to object $B$ is the rate of change of position of the object $A$ with respect to object $B$.
If $V_A$ and $V_B$ be the velocities of objects $A$ and $B$ with respect to the ground, then
The relative velocity of $A$ with respect to $B$ is $V_{AB} = V_A – V_B$
The relative velocity of $B$ with respect to $A$ is $V_{BA} = V_B – V_A$
So, it is clear that the motion of objects vary with frame of reference of different observers. And also if object $A$ or $B$ don't exist, then how will we be able to mention that phenomenon? The observer observes the motion of object $A$ and $B$. So it is said that motion is a combined property of the object under study and the observer.
Now the second one. Suppose you were sent to space within a rocket and you are moving with a constant velocity with respect to earth. In this frame of reference earth is the observer. But what will happen if there was nothing around you? It seems there is no observer to state your motion but it's not true. You are yourself an observer and you are at rest in your frame of reference. If there is no viewer, it means that nothing exist there. If there is motion there must be the presence of an observer. So, there is no meaning of rest or motion without the viewer.
Hope this helps.
A: It is just saying that all motion is measured relative to something else that could also be moving since there is no absolute reference for position in the universe. Though I think there was supposed to be something special about the CMB.
So it's just saying that you always need something to measure velocity against (velocity, not acceleration).
A: I think the second sentence is nonsense if taken literally. The temperature and pressure of the air in my room is the result of the kinetic energy- ie the motion- of the air molecules. You do not need to posit the existence of an observer to understand that.
A: It's basically saying that if you are to view an object and it's behavior, then you need to view it through a certain kind of 'window'. This window here is a frame of reference.
So, suppose your window is dirty then that will make the surrounding look dirty as well. In a real life case, you may actually clean the window and see through it, or, you may open the window and see through, but in the context of physics, you can't do neither. You must always view it in a frame of reference, there is no other way.
Due to the necessity of the choice of a frame, when observing an object, whatever peculiarities the frame acts will ultimately be blended into the observation we make. You'll see more of this when you learn concepts such as pseudo forces and accelerated frames of reference.
More on this:
However, it may also be noted, that there are some observer "agnostic" quantities. In sense, these particular quantities, you will measure the same no matter what frame you observe them in. Example of these could be things like air pressure, temperature, Newtonian time, charge etc (*)
For these particular quantities, you could perhaps drop the requirement of observation dependence that I mentioned initially... but I feel it is still pointless because if you are to observe them, then you need to actually go and do an experiment, so when you do it, you are implicitly choosing a frame of reference still.
*: If we are talking about relativity, then what time you experience depends on how fast you are moving. The thing invariant here is actually speed of light.
A: A non accelerating particle can always be said to have a velocity of zero in its own non accelerating frame of reference regardless of what an external viewer measures. In this manner, There is no meaning of rest or motion without a viewer of the particle in the viewers frame of reference.
