enses. Furthermore there are different ways of
making these abstractions which we think of as space and as time; and
under some circumstances we adopt one way and under other circumstances
we adopt another way. Thus there is no paradox in holding that what we
mean by space under one set of circumstances is not what we mean by
space under another set of circumstances. And equally what we mean by
time under one set of circumstances is not what we mean by time under
another set of circumstances. 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. You cannot recognise an event; because when it is gone, it
is gone. You may observe another event of analogous character, but the
actual chunk of the life of nature is inseparable from its unique
occurrence. But a character of an event can be recognised. We all know
that if we go to the Embankment near Charing Cross we shall observe an
event having the character which we recognise as Cleopatra's Needle.
Things which we thus recognise I call objects. An object is situated in
those events or in that stream of events of which it expresses the
character. There are