By saying that space and time are abstractions, I do not mean that they do not express for us real facts about nature.

What I mean is that there are no spatial facts or temporal facts apart from physical nature, namely that space and time are merely ways of expressing certain truths about the relations between events.
Also that under different circumstances there are different sets of truths about the universe which are naturally presented to us as statements about space.

In such a case what a being under the one set of circumstances means by space will be different from that meant by a being under the other set of circumstances.

Accordingly when we are comparing two observations made under different circumstances we have to ask 'Do the two observers mean the same thing by space and the same thing by time ?' The modern theory of relativity has arisen because certain perplexities as to the concordance of certain delicate observations such as the motion of the earth through the ether, the perihelion of mercury, and the positions of the stars in the neighbourhood of the sun, have been solved by reference to this purely relative significance of space and time.
I want now to recall your attention to Cleopatra's Needle, which I have not yet done with.

As you are walking along the Embankment you suddenly look up and say, 'Hullo, there's the Needle.' In other words, you recognise it.