1958/Heisenberg/11

==The Role of Modern Physics==
: in the Present Development of Human Thinking
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THE philosophical implications of modern physics have been 
discussed in the foregoing chapters in order to show that this 
most modern part of science touches very old trends of thought 
at many points, that it approaches some of the very old problems 
from a new direction. It is probably true quite generally that in 
the history of human thinking the most fruitful developments 
frequently take place at those points where two different lines of 
thought meet. These lines may have their roots in quite different 
parts of human culture, in different times or different cultural 
environments or different religious traditions: hence if they 
actually meet, that is, if they are at least so much related to each 
other that a real interaction can take place, then one may hope 
that new and interesting developments will follow. Atomic 
physics as a part of modern science does actually penetrate in our 
time into very different cultural traditions. It is not only taught 
in Europe and the Western countries, where it belongs to the 
traditional activity in the natural sciences, but it is also studied 
in the Far East, in countries like Japan and China and India, 
with their quite different cultural background, and in Russia, 
where a new way of thinking has been established in our time; a 
new way related both to specific scientific developments of the 
Europe of the nineteenth century and to other entirely different 
traditions from Russia itself. It can certainly not be the purpose 
of the following discussion to make predictions about the prob- 
able result of the encounter between the ideas of modern physics 
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and the older traditions. But it may be possible to define the points 
from which the interaction between the different ideas may begin. 
In considering this process of expansion of modern physics it 
would certainly not be possible to separate it from the general 
expansion of natural science, of industry and engineering, of 
medicine, etc., that is, quite generally of modern civilization in 
all parts of the world. Modern physics is just one link in a long 
chain of events that started from the work of Bacon, Galileo and 
Kepler and from the practical application of natural science in 
the seventeenth and eighteenth centuries. The connection be- 
tween natural science and technical science has from the begin- 
ning been that of mutual assistance: The progress in technical 
science, the improvement of the tools, the invention of new 
technical devices have provided the basis for more, and more 
accurate, empirical knowledge of nature; and the progress in the 
understanding of nature and finally the mathematical formula- 
tion of natural laws have opened the way to new applications of 
this knowledge in technical science. For instance, the invention 
of the telescope enabled the astronomers to measure the motion 
of the stars more accurately than before; thereby a considerable 
progress in astronomy and in mechanics was made possible. On 
the other hand, precise knowledge of the mechanical laws was 
of the greatest value for the improvement of mechanical tools, 
for the construction of engines, etc. The great expansion of this 
combination of natural and technical science started when one 
had succeeded in putting some of the forces of nature at the dis- 
posal of man. The energy stored up in coal, for instance, could 
then perform some of the work which formerly had to be done 
by man himself. The industries growing out of these new possi- 
bilities could first be considered as a natural continuation and 
expansion of the older trades; at many points the work of the 
machines still resembled the old handicraft and the work in 
the chemical factories could be considered as a continuation of 
the work in the dyehouses and the pharmacies of the older times. 
But later entirely new branches of industry developed which had 
no counterpart in the older trades; for instance, electrical engi- 
neering. The penetration of science into the more remote parts 
of nature enabled the engineers to use forces of nature which in 
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former periods had scarcely been known; and the accurate 
knowledge of these forces in terms of a mathematical formula- 
tion of the laws governing them formed a solid basis for the con- 
struction of all kinds of machinery. 
The enormous success of this combination of natural and 
technical science led to a strong preponderance of those nations 
or states or communities in which this kind of human activity 
flourished, and as a natural consequence this activity had to be 
taken up even by those nations which by tradition would not 
have been inclined toward natural and technical sciences. The 
modern means of communication and of traffic finally com- 
pleted this process of expansion of technical civilization. Un- 
doubtedly the process has fundamentally changed the conditions 
of life on our earth; and whether one approves of it or not, 
whether one calls it progress or danger, one must realize that it 
has gone far beyond any control through human forces. One 
may rather consider it as a biological process on the largest scale 
whereby the structures active in the human organism encroach 
on larger parts of matter and transform it into a state suited for 
the increasing human population. 
Modern physics belongs to the most recent parts of this de- 
velopment, and its unfortunately most visible result, the inven- 
tion of nuclear weapons, has shown the essence of this develop- 
ment in the sharpest possible light. On the one hand, it has 
demonstrated most clearly that the changes brought about by 
the combination of natural and technical sciences cannot be 
looked at only from the optimistic viewpoint; it has at least 
partly justified the views of those who had always warned 
against the dangers of such radical transmutation of our natural 
conditions of life. On the other hand, it has compelled even 
those nations or individuals who tried to keep apart from these 
dangers to pay the strongest attention to the new development, 
since obviously political power in the sense o f military power 
rests upon the possession of atomic weapons. It can certainly not 
be the task of this volume to discuss extensively the political im- 
plication of nuclear physics. But at least a few words may be 
said about these problems because they always come first into the 
minds of people when atomic physics is mentioned. 
It is obvious that the invention of the new weapons, especially 
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of the thermonuclear weapons, has fundamentally changed the 
political structure of the world. Not only has the concept of inde- 
pendent nations or states undergone a decisive change, since any 
nation which is not in possession of such weapons must depend 
in some way on those very few nations that do produce these 
arms in large quantity; but also the attempt of warfare on a 
large scale by means of such weapons has become practically an 
absurd kind of suicide. Hence one frequently hears the optimistic 
view that therefore war has become obsolete, that it will not 
happen again. This view, unfortunately, is a much too optimistic 
oversimplification. On the contrary, the absurdity of warfare by 
means of thermonuclear weapons may, in a first approximation, 
act as an incentive for war on a small scale. Any nation or 
political group which is convinced of its historical or moral right 
to enforce some change of the present situation will feel that the 
use of conventional arms for this purpose will not involve any 
great risks; they will assume that the other side will certainly not 
have recourse to the nuclear weapons, since the other side being 
historically and morally wrong in this issue will not take the 
chance of war on a large scale. This situation would in turn 
induce the other nations to state that in case of'small wars in- 
flicted upon them by aggressors, they would actually have re- 
course to the nuclear weapons, and thus the danger obviously 
remains. It may quite well be that in about twenty or thirty years 
from now the world will have undergone so great changes that 
the danger of warfare on a large scale, of the application of all 
technical resources for the annihilation of the opponent, will 
have greatly diminished or disappeared .But the way to this new 
state will be full of the greatest dangers. We must as in all former 
times, realize that what looks historically or morally right to the 
one side may look wrong to the other side. The continuation of 
the status quo may not always be the correct solution; it may, 
on the contrary, be most important to find peaceful means of 
adjustments to new situations, and it may in many cases be 
extremely difficult to find any just decision at all. Therefore, it is 
probably not too pessimistic to say that the great war can be 
avoided only if all the different political groups are ready to 
renounce some of their apparently most obvious rights — in view 
of the fact that the question of right or wrong may look essen- 
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tially different from the other side. This is certainly not a new 
point of view; it is in fact only an application of that human 
attitude which has been taught through many centuries by some 
of the great religions. 
The invention of nuclear weapons has also raised entirely new 
problems for science and scientists. The political influence of 
science has become very much stronger than it was before World 
War II, and this fact has burdened the scientist, especially the 
atomic physicist, with a double responsibility. He can either 
take an active part in the administration of the country in con- 
nection with the importance of science for the community; then 
he will eventually have to face the responsibility for decisions of 
enormous weight which go far beyond the small circle of re- 
search and university work to which he was wont. Or he may 
voluntarily withdraw from any participation in political de- 
cisions; then he will still be responsible for wrong decisions 
which he could possibly have prevented had he not preferred the 
quiet life of the scientist. Obviously it is the duty of the scientists 
to inform their governments in detail about the unprecedented 
destruction that would follow from a war with thermonuclear 
weapons. Beyond that, scientists are frequently requested to par- 
ticipate in solemn resolution in favour of world peace; but con- 
sidering this latter demand I must confess that I have never been 
able to see any point in declarations of this kind. Such resolutions 
may seem a welcome proof of goodwill; but anyone who speaks 
in favour of peace without stating precisely the conditions ot this 
peace must at once be suspected of speaking only about that 
kind of peace in which he and his group thrive best— which ot 
course would be completely worthless. Any honest declaration 
for peace must be an enumeration of the sacrifices one is pre- 
pared to make for its preservation. But as a rule the scientists 
have no authority to make statements of this kind. 
At the same time the scientist can do his best to promote 
international co-operation in his own field. The great importance 
that many governments attach to research in nuclear physics 
nowadays and the fact that the level of scientific work is still very 
different in different countries favours international co-opera- 
tion in this work. Young scientists of many different countries 
may gather in research institutions in which a strong activity in 
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the field of modern physics is going on and the common work on 
difficult scientific problems will foster mutual understanding. In 
one case, that of the Geneva organization, it has even been pos- 
sible to reach an agreement between a number of different na- 
tions for building a common laboratory and for constructing by a 
combined effort the expensive experimental equipment for re- 
search in nuclear physics. This kind of co-operation will certainly 
help to establish a common attitude toward the problems of 
science— common even beyond the purely scientific problems— 
among the younger generation of scientists. Of course one does 
not know beforehand what will grow out of the seeds that have 
been sown in this way when the scientists return into their old 
environments and again take part in their own cultural tradi- 
tions. But one can scarcely doubt that the exchange of ideas be- 
tween young scientists of different countries and between the dif- 
ferent generations in every country will help to approach with- 
out too much tension that new state of affairs in which a balance 
is reached between the older traditional forces and the inevitable 
necessities of modern life. It is especially one feature of science 
which makes it more than anything else suited for establishing 
the first strong connection between different cultural traditions. 
This is the fact that the ultimate decisions about the value of a 
special scientific work, about what is' correct or wrong in the 
work, do not depend on any human authority. It may sometimes 
take many years before one knows the solution of a problem, 
before one can distinguish between truth and error; but finally 
the questions will be decided, and the decisions are made not by 
any group of scientists but by nature itself. Therefore, scientific 
ideas spread among those who are interested in science in an 
entirely different way from the propagation of political ideas. 
While political ideas may gain a convincing influence among 
great masses of people just because they correspond or seem to 
correspond to the prevailing interests of the people, scientific 
ideas will spread only because they are true. They are objective 
and final criteria assuring the correctness of a scientific state- 
ment. 
All that has here been said about international co-operation 
and exchange of ideas would of course be equally true for any 
part of modern science; it is by no means confined to atomic 
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physics. In this respect modern physics is just one of the many 
branches of science, and even if its technical applications— the 
arms and the peaceful use of atomic energy— attach a special 
weight to this branch, there would be no reason for considering 
international co-operation in this field as far more important 
than in any other field. But we have now to discuss again those 
features of modern physics which are essentially different from 
the previous development of natural science, and we have for 
this purpose once more to go back to the European history of 
this development that was brought about by the combination of 
natural and technical sciences. 
It has frequently been discussed among the historians whether 
the rise of natural science after the sixteenth century was in any 
way a natural consequence of earlier trends in human thinking. 
It may be argued that certain trends in Christian philosophy led 
to a very abstract concept of God, that they put God so far 
above the world that one began to consider the world without at 
the same time also seeing God in the world. The Cartesian parti- 
tion may be called a final step in this development. Or one may 
point out that all the theological controversies of the sixteenth 
century produced a general discontent about problems that 
could not really be settled by reason and were exposed to the 
political struggles of the time; that this discontent favored 
interest in problems which were entirely separated from the 
theological disputes. Or one may simply refer to the enormous 
activity, the new spirit that had come into the European societies 
through the Renaissance. In any case during this period a new 
authority appeared which was completely independent of Chris- 
tian religion or philosophy or of the Church, the authority ot 
experience, of the empirical fact. One may trace this authority 
back into old philisophical trends, for instance, into the phi- 
losophy of Occam and Duns Scotus, but it became a vital 
force of human activity only from the sixteenth century onward 
Galileo did not only think about the mechanical motions, tne 
pendulum and the falling stone; he tried out by ™^ n ™:*}*' 
quantitatively, how these motions took Pk<^r"7fS£ 
was in its beginning certainly not meant as a deviation from the 
traditional Christian religion. On the contrary, °£^ f ^ 
kinds of revelation of God. The one was written in the Bible and 
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the other was to be found in the book of nature. The Holy Scrip- 
ture had been written by man and was therefore subject to error, 
while nature was the immediate expression of God's intentions. 
However, the emphasis on experience was connected with a 
slow and gradual change in the aspect of reality. While in the 
Middle Ages what we nowadays call the symbolic meaning of a 
thing was in some way its primary reality, the aspect of reality 
changed toward what we can perceive with our senses. What we 
can see and touch became primarily real. And this new concept 
of reality could be connected with a new activity: we can experi- 
ment and see how things really are. It was easily seen that this 
new attitude meant the departure of the human mind into an 
immense field of new possibilities, and it can be well understood 
that the Church saw in the new movement the dangers rather 
than the hopes. The famous trial of Galileo in connection with 
his views on the Copernican system marked the beginning of a 
struggle that went on for more than a century. In this contro- 
versy the representatives of natural science could argue that ex- 
perience offers an undisputable truth, that it cannot be left to 
any human authority to decide about what really happens in 
nature, and that this decision is made by nature or in this sense 
by God. The representatives of the traditional religion, on the 
other hand, could argue that by paying too much attention to 
the material world, to what we perceive with our senses, we lose 
the connection with the essential values of human life, with just 
that part of reality which is beyond the material world. These 
two arguments do not meet, and therefore the problem could 
not be settled by any kind of agreement or decision. 
In the meantime natural science proceeded to get a clearer 
and wider picture of the material world. In physics this picture 
was to be described by means of those concepts which we now- 
adays call the concepts of classical physics. The world consisted 
of things in space and time, the things consist of matter, and 
matter can produce and can be acted upon by forces. The events 
follow from the interplay between matter and forces; every event 
is the result and the cause of other events. At the same time the 
human attitude toward nature changed from a contemplative 
one to the pragmatic one. One was not so much interested in 
nature as it is; one rather asked what one could do with it. 
Therefore, natural science turned into technical science; every 
advancement of knowledge was connected with the question as 
to what practical use could be derived from it. This was true not 
only in physics; in chemistry and biology the attitude was essen- 
tially the same, and the success of the new methods in medicine 
or in agriculture contributed essentially to the propagation of the 
new tendencies. 
In this way, finally, the nineteenth century developed an 
extremely rigid frame for natural science which formed not only 
science but also the general outlook of great masses of people. 
This frame was supported by the fundamental concepts of clas- 
sical physics, space, time, matter and causality; the concept of 
reality applied to the things or events that we could perceive by 
our senses or that could be observed by means of the refined tools 
that technical science had provided. Matter was the primary 
reality The progress of science was pictured as a crusade ot 
conquest into the material world. Utility was the watchword of 
the time. . 
On the other hand, this frame was so narrow and rigid that it 
was difficult to find a place in it for many concepts of our lan- 
guage that had always belonged to its very substance, for 
instance, the concepts of mind, of the human soul or of life. 
Mind could be introduced into the general picture only as a kind 
of mirror of the material world; and when one studied the 
properties of this mirror in the science of psychology, the scien- 
tists were always tempted — if I may carry the comparison 
further— to pay more attention to its mechanical than to its 
optical properties. Even there one tried to apply the concepts of 
classical physics, primarily that of causality. In the same way lite 
was to be explained as a physical and chemical process, governed 
by natural laws, completely determined by causality. Darwin s 
concept of evolution provided ample evidence for this interpreta- 
tion It was specially difficult to find in this framework room 
for those parts of reality that had been the object of the tradi- 
ional religion and seemed now more or less only imaginary. 
Therefore in those European countries in which one was wont 
to follow the ideas up to their extreme consequences, an open 
hostility of science toward religion developed, and even in the 
other countries there was an increasing tendency toward in- 
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difference toward such questions; only the ethical values of the 
Christian religion were excepted from this trend, at least for the 
time being. Confidence in the scientific method and in rational 
thinking replaced all other safeguards of the human mind. 
Coming back now to the contributions of modern physics 
one may say that the most important change brought about by 
its results consists in the dissolution of this rigid frame of con- 
cepts of the nineteenth century. Of course many attempts had 
been made before to get away from this rigid frame which 
seemed obviously too narrow for an understanding of the essen- 
tial parts of reality. But it had not been possible to see what 
could be wrong with the fundamental concepts like matter 
space, time and causality that had been so extremely successful 
in the history of science. Only experimental research itself, car- 
ried out with all the refined equipment that technical science 
could offer, and its mathematical interpretation, provided the 
basis for a critical analysis— or, one may say, enforced the critical 
analysis— of these concepts, and finally resulted in the dissolu- 
tion of the rigid frame. 
This dissolution took place in two distinct stages. The first 
was the discovery, through the theory of relativity, that even 
such fundamental concepts as space and time could be changed 
and in fact must be changed on account of new experience. This 
change did not concern the somewhat vague concepts of space 
and time in natural language; but it did concern their precise 
formulation in the scientific language of Newtonian mechanics 
which had erroneously been accepted as final. The second stage 
was the discussion of the concept of matter enforced by the ex- 
perimental results concerning the atomic structure. The idea of 
the reality of matter had probably been the strongest part in that 
rigid frame of concepts of the nineteenth century, and this idea 
had at least to be modified in connection with the new ex- 
perience. Again the concepts so far as they belonged to the 
natural language remained untouched. There was no difficulty 
in speaking about matter or about facts or about reality when 
one had to describe the atomic experiments and their results. But 
the scientific extrapolation of these concepts into the smallest 
parts of matter could not be done in the simple way suggested 
by classical physics, though it had erroneously determined the 
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general outlook on the problem of matter. 
These new results had first of all to be considered as a serious 
warning against the somewhat forced application of scientific 
concepts in domains where they did not belong. The application 
of the concepts of classical physics, e.g., in chemistry, had been 
a mistake. Therefore, one will nowadays be less inclined to 
assume that the concepts of physics, even those of quantum 
theory, can certainly be applied everywhere in biology or other 
sciences. We will, on the contrary, try to keep the doors open 
for the entrance of new concepts even in those parts of science 
where the older concepts have been very useful for the under- 
standing of the phenomena. Especially at those points where the 
application of the older concepts seems somewhat forced or 
appears not quite adequate to the problem we will try to avoid 
any rash conclusions. 
Furthermore, one of the most important features of the de- 
velopment and the analysis of modern physics is the experience 
that the concepts of natural language, vaguely defined as they 
are, seem to be more stable in the expansion of knowledge than 
the precise terms of scientific language, derived as an idealization 
from only limited groups of phenomena. This is in fact not sur- 
prising since the concepts of natural language are formed by the 
immediate connection with reality; they represent reality. It is 
true that they are not very well defined and may therefore also 
undergo changes in the course of the centuries, just as reality 
itself did, but they never lose the immediate connection with 
reality. On the other hand, the scientific concepts are idealiza- 
tions; they are derived from experience obtained by refined 
experimental tools, and are precisely defined through axioms 
and definitions. Only through these precise definitions is it pos- 
sible to connect the concepts with a mathematical scheme and to 
derive mathematically the infinite variety of possible phenomena 
in this field. But through this process of idealization and precise 
definition the immediate connection with reality is lost. The con- 
cepts still correspond very closely to reality in that part of nature 
which had been the object of the research. But the correspond- 
ence may be lost in other parts containing other groups of 
phenomena. 
Keeping in mind the intrinsic stability of the concepts of 
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natural language in the process of scientific development, one 
sees that— after the experience of modern physics— our attitude 
toward concepts like mind or the human soul or life or God will 
be different from that of the nineteenth century, because these 
concepts belong to the natural language and have therefore 
immediate connection with reality. It is true that we will also 
realize that these concepts are not well defined in the scientific 
sense and that their application may lead to various contradic- 
tions, for the time being we may have to take the concepts 
unanalyzed as they are; but still we know that they touch reality! 
It may be useful in this connection to remember that even in the 
most precise part of science, in mathematics, we cannot avoid 
using concepts that involve contradictions. For instance it is 
well known that the concept of infinity leads to contradictions 
that have been analyzed, but it would be practically impossible 
to construct the main parts of mathematics without this concept. 
The general trend of human thinking in the nineteenth cen- 
tury had been toward an increasing confidence in the scientific 
method and in precise rational terms, and had led to a general 
scepticism with regard to those concepts of natural language 
which do not fit into the closed frame of scientific thought-for 
instance, those of religion. Modern physics has in many ways in- 
creased this skepticism; but it has at the same time turned it 
against the overestimation of precise scientific concepts, against 
scepticism itself. The scepticism against precise scientific con- 
cepts does not mean that there should be a definite limitation for 
the application of rational thinking. On the contrary, one may 
say that the human ability to understand may be in a certain 
sense unlimited. But the existing scientific concepts cover always 
only a very limited part of reality, and the other part that has 
not yet been understood is infinite. Whenever we proceed from 
the known into the unknown we may hope to understand but 
we may have to learn at the same time a new meaning of the 
word 'understanding'. We know that any understanding must 
be based finally upon the natural language because it is only 
there that we can be certain to touch reality, and hence we must 
be sceptical about any scepticism with regard to this natural 
language and its essential concepts. Therefore, we may use these 
concepts as they have been used at all times. In this way modern 
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physics has perhaps opened the door to a wider outlook on the 
relation between the human mind and reality. 
This modern science, then, penetrates in our time into other 
parts of the world where the cultural tradition has been entirely 
different from the European civilization. There the impact of 
this new activity in natural and technical science must make 
itself felt even more strongly than in Europe, since changes in 
the conditions of life that have taken two or three centuries in 
Europe will take place there within a few decades. One should 
expect that in many places this new activity must appear as a 
decline of the older culture, as a ruthless and barbarian attitude, 
that upsets the sensitive balance on which all human happiness 
rests. Such consequences cannot be avoided; they must be taken 
as one aspect of our time. But even there the openness of modern 
physics may help to some extent to reconcile the older traditions 
with the new trends of thought. For instance, the great scientific 
contribution in theoretical physics that has come from Japan 
since the last war may be an indication for a certain relationship 
between philosophical ideas in the tradition of the Far East and 
the philosophical substance of quantum theory. It may be easier 
to adapt oneself to the quantum-theoretical concept of reality 
when one has not gone through the naive materialistic way of 
thinking that still prevailed in Europe in the first decades of this 
century. 
Of course such remarks should not be misunderstood as an 
underestimation of the damage that may be done or has been 
done to old cultural traditions by the impact of technical prog- 
ress. But since this whole development has for a long time passed 
far beyond any control by human forces, we have to accept it as 
one of the most essential features of our time and must try to 
connect it as much as possible with the human values that have 
been the aim of the older cultural and religious traditions. It may 
be allowed at this point to quote a story from the Hasidic re- 
ligion: There was an old rabbi, a priest famous for his wisdom, 
to whom all people came for advice. A man visited him in 
despair over all the changes that went on around him, deploring 
all the harm done by so-called technical progress. 'Isn't all this 
technical nuisance completely worthless,' he exclaimed, 'if one 
considers the real values of life?' 'This may be so,' the rabbi 
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replied, 'but if one has the right attitude one can learn from 
everything.' 'No,' the visitor rejoined, 'from such foolish things 
as railway or telephone or telegraph one can learn nothing 
whatsoever.' But the rabbi answered, 'You are wrong. From 
the railway you can learn that you may by being one instant late 
miss everything. From the telegraph you can learn that every 
word counts. And from the telephone you can learn that what 
we say here can be heard there.' The visitor understood what the 
rabbi meant and went away. 
Finally, modern science penetrates into those large areas of 
our present world in which new doctrines were established only 
a few decades ago as foundations for new and powerful societies. 
There modern science is confronted both with the content of the 
doctrines, which go back to European philosophical ideas of the 
nineteenth century (Hegel and Marx), and with the phe- 
nomenon of uncompromising belief. Since modern physics must 
play a great role in these countries because of its practical ap- 
plicability, it can scarcely be avoided that the narrowness of the 
doctrines is felt by those who have really understood modern 
physics and its philosophical meaning. Therefore, at this point 
an interaction between science and the general trend of thought 
may take place. Of course the influence of science should not be 
overrated; but it might be that the openness of modern science 
could make it easier even for larger groups of people to see that 
the doctrines are possibly not so important for the society as had 
been assumed before. In this way the influence of modern 
science may favour an attitude of tolerance and thereby may 
prove valuable. 
On the other hand, the phenomenon of uncompromising belief 
carries much more weight than some special philosophical no- 
tions of the nineteenth century. We cannot close our eyes to the 
fact that the great majority of the people can scarcely have any 
well-founded judgment concerning the correctness of certain 
important general ideas or doctrines. Therefore, the word 
'belief can for this majority not mean 'perceiving the truth of 
something' but can only be understood as 'taking this as the 
basis for life'. One can easily understand that this second kind 
of belief is much firmer, is much more fixed than the first one. 
that it can persist even against immediate contradicting ex- 
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perience and can therefore not be shaken by added scientific 
knowledge. The history of the past two decades has shown by 
many examples that this second kind of belief can sometimes be 
upheld to a point where it seems completely absurd, and that it 
then ends only with the death of the believer. Science and history 
can teach us that this kind of belief may become a great danger 
for those who share it. But such knowledge is of no avail, since 
one cannot see how it could be avoided, and therefore such belief 
has always belonged to the great forces in human history. From 
the scientific tradition of the nineteenth century one would of 
course be inclined to hope that all belief should be based on a 
rational analysis of every argument, on careful deliberation; and 
that this other kind of belief, in which some real or apparent 
truth is simply taken as the basis for life, should not exist. It is 
true that cautious deliberation based on purely rational argu- 
ments can save us from many errors and dangers, since it allows 
readjustment to new situations, and this may be a necessary 
condition for life. But remembering our experience in modern 
physics it is easy to see that there must always be a fundamental 
complementarity between deliberation and decision. In the prac- 
tical decisions of life it will scarcely ever be possible to go 
through all the arguments in favour of or against one possible 
decision, and one will therefore always have to act on insufficient 
evidence. The decision finally takes place by pushing away all 
the arguments — both those that have been understood and 
others that might come up through further deliberation — and 
by cutting off all further pondering. The decision may be the 
result of deliberation, but it is at the same time complementary 
to deliberation; it excludes deliberation. Even the most important 
decisions in life must always contain this inevitable element of 
irrationality. The decision itself is necessary, since there must 
be something to rely upon, some principle to guide our actions. 
Without such a firm stand our own actions would lose all force. 
Therefore, it cannot be avoided that some real or apparent truth 
form the the basis of life; and this fact should be acknowledged 
with regard to those groups of people whose basis is different 
from our own. 
Coming now to a conclusion from all that has been said about 
modern science, one may perhaps state that modern physics is 
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just one, but a very characteristic, part of a general historical 
process that tends toward a unification and a widening of our 
present world. This process would in itself lead to a diminution 
of those cultural and political tensions that create the great 
danger of our time. But it is accompanied by another process 
which acts in the opposite direction. The fact that great masses 
of people become conscious of this process of unification leads to 
an instigation of all forces in the existing cultural communities 
that try to ensure for their traditional values the largest possible 
role in the final state of unification. Thereby the tensions increase 
and the two competing processes are so closely linked with each 
other that every intensification of the unifying process— for in- 
stance, by means of new technical progress— intensifies also the 
struggle for influence in the final state, and thereby adds to the 
instability of the transient state. Modern physics plays perhaps 
only a small role in this dangerous process of unification. But it 
helps at two very decisive points to guide the development into 
a calmer kind of evolution. First, it shows that the use of arms 
in the process would be disastrous and, second, through its open- 
ness for all kinds of concepts it raises the hope that in the final 
state of unification many different cultural traditions may live 
together and may combine different human endeavours into a f 
new kind of balance between thought and deed, between activity 
and meditation. 
==Notes==
* http://archive.org/stream/PhysicsPhilosophy/Heisenberg-PhysicsPhilosophy_djvu.txt