increase in the types of objects directly sensed. Deli- cacy of sense-apprehension means perceptions of objects as distinct entities which are mere subtle ideas to cruder sensibilities. The phrasing of music is a mere abstract subtlety to the unmusical; it is a direct sense-appre- hension to the initiated. For example, if we could imagine some lowly type of organic being thinking and aware of our thoughts, it would wonder at the abstract subtleties in which we indulge as we think of stones and bricks and drops of water and plants. It only knows of vague undifferentiated feelings in nature. It would consider us as given over to the play of excessively abstract intellects. But then if it could think, it would anticipate; and if it anticipated, it would soon per- ceive for itself.
In these lectures we have been scrutinising the
foundations of natural philosophy. We are stopping at
the very point where a boundless ocean of enquiries
opens out for our questioning.
I agree that the view of Nature which I have main-
tained in these lectures is not a simple one. Nature
appears as a complex system whose factors are dimly
discerned by us. But, as I ask you. Is not this the very
truth ? Should we not distrust the jaunty assurance with
which every age prides itself that it at last has hit upon
the ultimate concepts in which all that happens can be
formulated ? The aim of science is to seek the simplest
explanations of complex facts. We are apt to fall into
the error of thinking that the facts are simple because
simplicity is the goal of our quest. The guiding motto
in the life of every natural philosopher should be,
Seek simplicity and distrust it.
Ii — 2
CHAPTER VIII SUMMARY
There is a general agreement that Einstein's investiga- tions have one fundamental merit irrespective of any criticisms which we may feel inclined to pass on them. They have made us think. But when we have admitted so far, we are most of us faced with a distressing per- plexity. What is it that we ought to think about? The purport of my lecture this afternoon will be to meet this difficulty and, so far as I am able, to set in a clear light the changes in the background of our scientific thought which are necessitated by any acceptance, however qualified, of Einstein's main positions. I remember that I am lecturing to the members of a chemical society who are not for the most part versed in advanced mathematics. The first point that I would urge upon you is that what immediately concerns you is not so much the detailed deductions of the new theory as this general change in the background of scientific conceptions which will follow from its acceptance. Of course, the detailed deductions are important, because unless our colleagues the astronomers and the physicists find these predictions to be verified we can neglect the theory altogether. But we may now take it as granted that in many striking particulars these deductions have been found to be in agreement with observation. Accord- ingly the theory has to be taken seriously and we are anxious to know what will be the consequences of its final acceptance. Furthermore during the last few weeks
CH. viii] SUMMARY 165
the scientific journals and the lay press have been filled with articles as to the nature of the crucial experiments which have been made and as to some of the more striking expressions of the outcome of the new theory. 'Space caught bending' appeared on the news-sheet of a well-known evening paper. This rendering is a terse but not inapt translation of Einstein's own way of interpreting his results. I should say at once that I am a heretic as to this explanation and that I shall expound to you another explanation based upon some work of my own, an explanation which seems to me to be more in accordance with our scientific ideas and with the whole body of facts which have to be explained. We have to remember that a new theory must take account of the old well-attested facts of science just as much as of the very latest experimental results which have led to its production.
To put ourselves in the position to assimilate and to criticise any change in ultimate scientific conceptions we must begin at the beginning. So you must bear with me if I commence by making some simple and obvious reflections. Let us consider three statements, (i) 'Yes- terday a man was run over on the Chelsea Embankment,' (ii) 'Cleopatra's Needle is on the Charing Cross Em- bankment,' and (iii) 'There are dark lines in the Solar Spectrum.' The first statement about the accident to the man is about what we may term an ' occurrence,' a 'happening,' or an 'event.' I will use the term * event ' because it is the shortest. In order to specify an observed event, the place, the time, and character of the event are necessary. In specifying the place and the time you are really stating the relation of the assigned event to the general structure of other observed events. For
i66 THE CONCEPT OF NATURE [CH.
example, the man was run over between your tea and
your dinner and adjacently to a passing barge in the
river and the traffic in the Strand. The point which I
want to make is this: Nature is known to us in our
experience as a complex of passing events. In this
complex we discern definite mutual relations between
component events, which we may call their relative
positions, and these positions we express partly in terms
of space and partly in terms of time. Also in addition
to its mere relative position to other events, each par-
ticular event has its own peculiar character. In other
words, nature is a structure of events and each event
has its position in this structure and its own peculiar
character or quality.
Let us now examine the other two statements in the light of this general principle as to the meaning of nature. Take the second statement, 'Cleopatra's Needle is on the Charing Cross Embankment.' At first sight we should hardly call this an event. It seems to lack the element of time or transitoriness. But does it ? If an angel had made the remark some hundreds of millions of years ago, the earth was not in existence, twenty millions of years ago there was no Thames, eighty years ago there was no Thames Embankment, and when I was a small boy Cleopatra's Needle was not there. And now that it is there, we none of us expect it to be eternal. The static timeless element in the rela- tion of Cleopatra's Needle to the Embankment is a pure illusion generated by the fact that for purposes of daily intercourse its emphasis is needless. What it comes to is this : Amidst the structure of events which form the medium within which the daily life of Lon- doners is passed we know how to identify a certain
Viii] Summary 167
stream of events which maintain permanence of charac- ter, namely the character of being the situations of Cleopatra's Needle. Day by day and hour by hour we can find a certain chunk in the transitory life of nature and of that chunk we say, ' There is Cleopatra's Needle.' If we define the Needle in a sufficiently abstract manner we can say that it never changes. But a physicist who looks on that part of the life of nature as a dance of electrons, will tell you that daily it has lost some mole- cules and gained others, and even the plain man can see that it gets dirtier and is occasionally washed. Thus the question of change in the Needle is a mere matter of definition. The more abstract your definition, the more permanent the Needle, But whether your Needle change or be permanent, all you mean by stating that it is situated on the Charing Cross Embankment, is that amid the structure of events you know of a certain con- tinuous limited stream of events, such that any chunk of that stream, during any hour, or any day, or any second, has the character of being the situation of Cleopatra's Needle.
Finally, we come to the third statement, ' There are
dark lines in the Solar Spectrum.' This is a law of nature.
But what does that mean ? It means merely this. If any
event has the character of being an exhibition of the
solar spectrum under certain assigned circumstances, it
will also have the character of exhibiting dark lines in
that spectrum.
This long discussion brings us to the final conclusion that the concrete facts of nature are events exhibiting a certain structure in their mutual relations and certain characters of their own. The aim of science is to express the relations between their characters in terms of the
i68 THE CONCEPT OF NATURE [CH.
mutual structural relations between the events thus
characterised. The mutual structural relations between
events are both spatial and temporal. If you think of
them as merely spatial you are omitting the temporal
element, and if you think of them as merely temporal
you are omitting the spatial element. Thus when you
think of space alone, or of time alone, you are dealing
in abstractions, namely, you are leaving out an essential
element in the life of nature as known to you in the
experience of your senses. Furthermore there are
different ways of making these abstractions which we
think of as space and as time ; and under some circum-
stances 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 circum-
stances. 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. Accord-
ingly when we are comparing two observations made
under different circumstances we have to ask 'Do the
Viii] Summary 169
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 con-
cordance 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 neigh-
bourhood 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 recog- nised. 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 many sorts of objects. For example, the colour green is an object according to the above definition. It is the purpose of science to trace the laws which govern the appearance of objects in the various events in which they are found to be situated. For this purpose we can mainly concentrate on two types of objects, which I will call material physical objects and scientific objects, A material physical object is an ordinary bit of matter, Cleopatra's Needle for example. This is a much more complicated type of object than a mere colour, such as
170 THE CONCEPT OF NATURE [CH.