Back to the Pre-Socratics: The Presidential Address
Author(s): Karl R. Popper
Reviewed work(s):
Source: Proceedings of the Aristotelian Society, New Series, Vol. 59 (1958 - 1959), pp. 1-24
Published by: Wiley on behalf of The Aristotelian Society
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PAPERS READ BEFORE THE SOCIETY 1958-59
Meeting of the AristotelianSociety at 21, BedfordSquare,,London,W.C.1,
on 13th October,1958, at 7.30 p.m.
I.-BACK
TO THE PRE-SOCRATICS
By KARLR. POPPER
THE PRESIDENTIAL ADDRESS
I
was a progressiveprogramme,comn'BACK TO METHUSELAH'
pared with 'Back to Thales': what Shaw offered us was an
improvedexpectationof life-something that was in the air, at
any rate when he wrote. I have nothing to offer you, I am
afraid,that is in the air to-day; for what I want to returnto is
the simple straightforward
rationalityof the Pre-Socratics. The
of
their
and
boldness
simplicity
questionsis part of it, but more
attitude
importantstill is the critical
which,as I shalltry to show,
was first developedin the Ionian School.
The questionswhich the Pre-Socraticstried to answerwere
primarily cosmological questions, but they also dealt with
questions of the theory of knowledge. It is my belief that
philosophymust returnto cosmologyand to a simpletheoryof
knowledge. There is at least one philosophicalproblem in
which all thinkingmen are interested: the problem of understandingthe worldin whichwe live, includingourselves,who are
part of that world, and our knowledgeof it. All science is
cosmology, I believe, and for me the interestof philosophyas
well as of sciencelies solely in their bold attemptto add to our
knowledgeof the world, and to the theoryof our knowledgeof
the world. I am interestedin Wittgenstein,for example,not
because of his linguisticphilosophy,but because his Tractatus
was a cosmologicaltreatise,and becausehis theoryof knowledge
was closelylinkedwith his cosmology.
For me, philosophyas well as sciencelose all attractionwhen
they give up that pursuit-when they become specialismsand
A
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KARL R. POPPER
cease to see, and to wonder at, the riddles of our world. Specialization may be a great temptation for the scientist. For the
philosopher it is the mortal sin.
II
In this paper I speak as an amateur, as a lover of the beautiful
story of the Pre-Socratics. I am not a specialist, nor an expert:
I am completely out of my depth when an expert begins to argue
what words or phrases Heraclitus might have used, and what
words or phrases he could not possibly have used. But when
the experts replace a beautiful story, based on the oldest texts we
possess, by one which-to me at any rate-makes no sense any
longer, then I feel that even an amateur may stand up and defend
an old tradition. Thus I will at least look into the experts'
arguments, and examine their consistency. This seems a harmless occupation to indulge in; and if an expert or anybody else
should take the trouble to refute my criticism, I shall be pleased
and honoured.
I shall be concerned with the cosmological theories of the
Pre-Socratics only to the extent to which they bear upon the
development of the problem of change, as I call it, and only to the
extent to which they are needed for an understanding of the
Pre-Socratic approach to the problem of knowledge-of their
practical approach as well as of their theoretical approach. For
it is of considerable interest to see how their practice as well as
their theory of knowledge is connected with the cosmological and
theological questions which they posed to themselves. Theirs
was not a theory of knowledge that began with the question
' How do I know that this is an orange?' or ' How do I know
that the object I am now perceiving is an orange?' Rather, their
theory of knowledge started from problems such as ' How do we
know that the world is made of water?' or 'How do we know
that the world is full of gods?' or 'How can we know anything
about the gods?'
There is a widespread belief, due I think to the somewhat
remote influence of Francis Bacon, that it is better to study the
problems of the theory of knowledge in connexion with our
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B3ACKTO THE PRE-SOCRATICS
knowledge of an orange rather than in connexion with our
knowledgeof the cosmos. I dissent from this belief, and it is
one of the main purposesof my paperto conveyto you some of
my reasonsfor dissenting. At any rate, it is good to remember
from time to time that our Westernscience-and thereseems to
be no other-did not startwithcollectingobservationsof oranges,
but with bold theoriesabout the world.
III
Traditional empiricist epistemology, and the traditional
historiographyof science, are still deeply influenced by the
Baconianmythaccordingto whichsciencestartsfromobservation
and then slowly and cautiouslyproceedsto theories. That the
facts are very differentcan be learnedfrom studyingthe early
Pre-Socratics. Herewe findfascinatingideas,some of whichare
strange and even staggering anticipationsof modern results
while many othersare, from our modernpoint of view, wide of
the mark; but most of them, and tne best of them,have nothing
to do with observation. Take for examplesome of the theories
about the shape and position of the earth. Thalessaid, we are
told, ' that the earthis supportedby wateron whichit rideslike
a ship, and when we say that there is an earthquake,then the
earth is being shaken by the movement of the water'. No
doubt, Thales had observedearthquakesas well as the rolling
of a ship before he arrivedat his theory. But the point of his
theorywas to explain the supportor suspensionof the earth,and
also earthquakes,by the conjecturethat the earthfloatson water;
and for this conjecture(whichso strangelyanticipatesthe modern
theory of continentaldrift) he could have no basis in his observations.
We mustnot forgetthat the functionof the Baconianmythis
to explainwhy scientificstatementsare true,by pointingout that
observationis the ' true source' of our scientific knowledge.
Once we realizethat all scientificstatementsare hypotheses,or
guesses, or conjectures,and that the vast majority of these
conjectures(includingBacon'sown) have turnedout to be false,
the Baconian myth becomes irrelevant. For it is pointless to
A 2
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itiL R. POPPER
argue that the conijecturesof science-those which have proved
to be false as well as those whichare still accepted-all startfrom
observation.
Howeverthis may be, Thales'beautifultheoryof the support
or suspensionof the earthand of earthquakes,thoughin no sense
based upon observation,is at least inspiredby an empiricalor
observationalanalogy. But even this is no longer true of the
theory proposed by Thales' great pupil, Anaximander.
Anaximander's
theoryof the suspensionof the earthis still highly
intuitive,but it no longeruses observationalanalogies. In fact,
it may be described as counter-observational.According to
Anaximander'stheory,'The earth... is held up by nothing,but
remainsstationaryowingto the fact thatit is equallydistantfrom
all otherthings. Its shapeis . .. like that of a drum. .. We walk
on one of its flatsurfaces,whilethe otheris on the oppositeside '.
The drum,of course,is an observationalanalogy. But the idea
of the earth'sfree suspensionin space,and the explanationof its
stability,has no analogywhateverin the wholefieldof observable
facts.
In my opinion, this idea of Anaximander'sis one of the
boldest, most revolutionary,and most portentousideas in the
whole historyof humanthought. It made possiblethe theories
of Aristarchusand Copernicus. But the step taken by Anaximanderwas evenmoredifficultand audaciousthanthe one taken
by Aristarchusand Copernicus. To envisagethe earthas freely
poised in mid-space,and to say ' that it remains motionless
because of its equilibrium' (as Aristotle paraphrasesAnaximander),is to anticipate,to some extent,even Newton'sidea of
immaterialand invisiblegravitationalforces.
IV
How did Anaximanderarrive at this remarkabletheory?
Certainlynot by observationbut by reasoning. And since his
theoryis an attemptto solve one of the problemsfor which his
teacher and kinsman Thales, the founder of the Milesian or
Ionian School, had offereda solution before him, I conjecture
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BACK TO THE PRE-SOCRATICS
5
that Anaximanderarrivedat his theory by way of criticising
Thales'theory. This conjecture,I believe,can be supportedby
a considerationof the structureof Anaximander'stheory.
For Anaximanderis likely to have argued against Thales'
theory (accordingto which the earth was floatingon water)on
the followinglines. Thales'theory is a specimenof a type of
theorywhich,if consistentlydeveloped,would lead to an infinite
regress. If we explain the stable position of the earth by the
assumptionthat it is supportedby water-that it is floatingon
the ocean (Okeanos)-should we not have to explainthe stable
position of the ocean by an analogous hypothesis? But this
wouldmeanthat we have to look for a supportof the ocean,and
then for a supportof this support. This methodof explanation
is unsatisfactory;first,becausewe solve our problemby creating
an exactly analogous one, and also for the less formal and
more intuitivereason that, in any such system of supportsor
props, any failureto secureone of the lower props must lead to
the collapseof the whole edifice.
From this we see intuitivelythat the stability of the world
cannotbe securedby a systemof supportsor props. In its place,
Anaximanderappealsto the internalor structuralsymmetryof
the world which ensuresthat there is no preferreddirectionin
which a collapsecan take place. He appliesthe principlethat
wherethere are no differences,therecan be no change. In this
way, he explainsthe stabilityof the earth by the equalityof its
distancesfrom all other things.
argument. It is important
This, it seems,was Anaximander's
that
it
even
to realize
though not quite consciously,
abolishes,
perhaps, and not quite consistently,the idea of an absolute
direction-the absolutesense of 'upwards' and 'downwards'.
Thisis not only contraryto all experiencebut notoriouslydifficult
to grasp. Anaximenesignored it, it seems, and even Anaximanderhimselfdid not graspit completely. For the idea of an
equal distancefrom all other things should have led him to the
theory that the earth has the shape of a globe. Instead, he
believedthat it had the shape of a drum,with an upperand a
lower flat surface. Yet it looks as if the remark' we walk on
one of its flat surfaces,while the other is on the opposite side'
containeda hint that therewas not an absolutelyuppersurface,
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KARL R. POPPER
but that, on the contrary,the surfaceon which we happenedto
walk was the one we mightcall the upper.
What preventedAnaximanderfrom arrivingat the theory
that the earth was a globe ratherthan a drum? There can be
littledoubtaboutthe answerto thisquestion: it was observational
experiencewhichtaughthim that the surfaceof the earthwas, by
and large,flat. Thus it was a speculativeand criticalargument,
the abstractcriticaldiscussionof Thales'theory,which almost
led him to the true theoryof the shapeof the earth; and it was
observationalexperiencewhichled him astray.
V
Thereis an obvious objectionagainstAnaximander'stheory
of symmetryaccordingto whichthe earthis equallydistantfrom
all otherthings: the asymmetryof the universecan be easilyseen
from the existenceof sun and moon, and especiallyfrom the fact
that sun and moon aresometimesnot far distantfromeachother,
so that they are on the same side of the earth, while there is
nothing on the other side to balance them. It appears that
Anaximandermet this objectionby another bold theory-his
theoryof the hiddennatureof the sun, the moon, and the other
heavenlybodies.
He envisagesthe rims of two huge chariot wheels rotating
round the earth, one 27 times the size of the earth, the other
18 timesits size. Each of theserims or circularpipesis filledby
throughwhichthefireis visible.
fire,andeachhas a breathing-hole
These holes we call, respectively,the sun and the moon. The
rest of the wheel is invisible,presumablybecauseit is dark (or
misty) and far away. The fixed stars (and presumablythe
planets)are also holes on wheels which are nearerto the earth
thanthe wheelsof the sunand the moon. Thewheelsof the fixed
starsrotaterounda commonaxis (whichwe now call the axis of
the earth) and togetherthey form a sphereround the earth, in
accordancewith the postulate that all things are positionedat
similardistancesfrom the earth. In this way, Anaximanderalso
becamethe founderof the theoryof spheres.
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VI
Therecan be no doubtwhateverthat Anaximander'stheories
are criticaland speculativeratherthanempirical:and considered
as approachesto truth, his critical and abstractspeculations
servedhim betterthan observationalexperience.
But this, a follower of Bacon may reply, is preciselywhy
Anaximanderwas not a scientist. Thisis preciselywhy we speak
of early Greek philosophyrather than of early Greek science.
Philosophy is speculative: everybody knows this. And as
everybodyknows,sciencebeginsonlywhenthespeculativemethod
is replacedby the observationalmethod, and when deduction
is replacedby induction.
This reply, of course, amounts to the thesis that scientific
theories should be defined by referenceto their origin-their
origin in observations,or in so-called ' inductiveprocedures'.
Yet I believethat few, if any, physicaltheorieswould fall under
this definition. And I do not see at all why the questionof origin
should be important. What is importantabout a theory is its
explanatorypower,and whetherit standsup to criticismand to
tests. The questionof its origin,of how it is arrivedat-whether
by an ' inductiveprocedure' as somesay, or by an act of intuition
-may be extremelyinterestingfrom a point of view of the
biographyof its originator,but it has little to do withits scientific
character.
VII
As to the Pre-Socratics,I assertthat thereis the most perfect
possible continuity of thought between their theories and the
later developments in physics. Whether they are called
philosophers,or pre-scientists,or scientistsmattersvery little, I
think. But I do assert that Anaximander'stheory broke the
way for the theories of Aristarchus,Copernicus,Kepler and
Galileo. It is not that he merely ' influenced' these latter
thinkers: 'influence' is a very superficialcategory. I would
ratherput it like this: Anaximander'sachievementis valuablein
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KARL R. POPPER
itself like a work of art. Besides,his achievementmade other
achievementspossible: amongthemthose of the greatscientists
mentioned.
But are not Anaximander'stheories false, and therefore
non-scientific? Theyarefalse,I admit,but so aremanytheories,
based upon countlessexperiments,which were held by modern
science until recently, and to which nobody would deny the
character of scientific theories, even though they are false.
(An exampleis the theorythat thereis one and only one kind of
atom-the lightest of all atoms-with the typical chemical
propertiesof hydrogen.) Therewere historiansof sciencewho
tendedto regardas unscientific(or evenas superstitious)any view
no longeracceptedat the time they werewriting; but this is an
untenableattitude. A false theorymay be as great an achievement as a true one. And many false theorieshave been more
helpfulin our searchfor truththan some less interestingtheories
which are still accepted. For false theories can be helpful in
manyways: theymaysuggestsomemoreor less radicalmodifications, and they may stimulatecriticism. Thus Thales'theory
that the earthfloats on waterreappearedin a modifiedform in
Anaxemines,and in more recenttimesin the form of Wegener's
famous theory of continental drift. How Thales' theory
stimulatedAnaximander'scriticismhas been shown already.
Anaximander's
theory,similarly,suggesteda modifiedtheorythe theory of an earth globe, freely poised in the centre of the
Universe,and surroundedby sphereson which heavenlybodies
were mounted. And by stimulatingcriticism,it also led to the
theorythatthe moon shiriesby reflectedlight; to the Pythagorean
theoryof a centralfire; and ultimatelyto the heliocentricworldsystemof Aristarchusand Copernicus.
VIII
I believe that the Milesians,like their orientalpredecessors
who took the worldfor a tent, envisagedthe world as a kind of
house, the home of all creatures-our home. Thus there was
no need to ask what it was for. But there was a real need to
inquireinto its architecture. The questionsof its structure,its
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9
ground-plan,and its building material, constitute the main
problemsof Milesian cosmology. There is also a speculative
interestin its origin, the question of cosmogony. It seems to
me that the cosmologicalinterestof the Milesiansfar exceeded
their cosmogonicalinterest,especiallyif we considerthe strong
cosmogonicaltradition,and the almost irresistibletendencyto
describea thing by describinghow it has been made, and thus
to present a cosmological account in a cosmogonical form.
The cosmological interest must be very strong, as compared
with the cosmogonicalone, if the presentationof a cosmological theory is even partially free from these cosmogonical
trappings.
I believethatit was Thaleswho firstdiscussedthe architecture
of the cosmos-its structure,ground-plan,and buildingmaterial.
In Anaximanderwe find answersto all threequestions. I have
brieflymentionedhis answerto the questionof structure. As to
the questionof the ground-planof the world,it was studiedand
expoundedby Anaximander,as indicatedby the traditionthat
he drew the first map of the world. And of course, he had a
theory of its building material-of 'the unformed' or 'the
unbounded' (the apeiron).
In Anaximander'sworld all kinds of changeswere going on.
Therewas a firewhichneededair and breathing-holes,
and these
were at times blocked up, so that the fire was smothered: this
was his theoryof eclipses,and of the phasesof the moon. There
werewinds,whichwereresponsiblefor the changingweather,and
indeedfor all otherchangeswithinthe cosmicedifice.
We haveherethe firsthint of whatwas soon to come: of the
problemof changewhich becamethe centralproblemof Greek
cosmology,and which led, with Leucippusand Democritus,to
a universaltheoryof changethat was acceptedby modernscience
almost up to the beginningof the twentiethcentury. (It was
given up only with the breakdownof Maxwell'smodels of the
ether,an historicalevent that was little noticedbefore 1905.)
IX
Theexcitingstoryof the developmentof theproblemof change
appearsto me in dangerof being completelyburiedunderthe
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KARL R. POPPER
mountingheap of the minutiaeof textual criticism. The story
cannot, of course,be fully told in one short paper,and still less
in one of its many sections. In briefest outline, the story is
this.
For Anaximander,our own world, our own cosmic edifice,
was only one of an infinityof worlds-an infinitywithoutbounds
in space and time. This system of worldswas eternal,and so
was motion. Therewas thusno needto explainmotion,no need
to constructa generaltheory of change. But there was a need
to explainthe well-knownchangesoccurringin our world. The
most obvious changes-the change of day and night, of the
winds and of the weather,and of the seasons, from sowing to
harvesting,and of the growthof plants and animalsand menwere all connectedwith the contrast of temperatures,with the
oppositionbetweenthe hot and the cold, and with that between
the dry and the wet. 'Living creaturescame into being from
moistureevaporatedby the sun', we are told; and the hot and
the cold also administerto the genesisof our own world edifice.
The hot and the cold were also responsiblefor the winds which
wereconceivedas the agentsof all change.
Anaximenes, a pupil of Anaximanderand his successor,
developedtheseideasin muchdetail. LikeAnaximanderhe was
interestedin the oppositionof the hot and the cold, of the moist
and the dry, and he explained the transitionsbetween these
opposites by a theory of condensationand rarefaction. Like
Anaximander,he believedin eternalmotion and in the action of
the winds; and it seems not unlikelythat one of the two main
points in which he deviatedfrom Anaximanderwas reachedby
a criticismof the idea that what was completelyboundlessand
formless (the apeiron) could be in motion. At any rate, he
replacedthe apeironby air-something thatwas almostboundless
and formless,and yet, accordingto Anaximander'sold theoryof
the winds, not only capable of motion, but the main agent of
motion and change. A similarunificationof ideas was achieved
by Anaximenes'theorythat ' the sun consistsof earth,and that
it gets very hot owing to the rapidity of its motion'. The
replacementof the more abstract theory of the unbounded
apeiron by the less abstractand more common-sensetheory of
air is matchedby the replacementof Anaximander'bold theory
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11
of the stabilityof the earthby the more common-senseidea that
the earth's ' flatnessis responsiblefor its stability; for it . . .
coverslike a lid the air beneathit '. Thus the earthrides on air
as the lid of a pot may ride on steam,or as a ship may ride on
water: Thales' question and Thales' answer are both reinstituted,and Anaximander'sepoch making argumentis not
understood. Anaximenes is an eclectic, a systematiser,an
empiricist,a manof commonsense. Of thethreegreatMilesians,
he is least creativeof new ideas; he is the least philosophically
minded.
The three Milesians all looked at our world as our home.
Therewas movement,therewas changein this home, there was
hot and cold, fireand moisture: it was exposedto the winds,and
a bit draughty,to be sure; but it was home,andit meantsecurity
and stabilityof a sort. But for Heraclitus,the house was on
fire.
There was no stability left in the world of Heraclitus.
'Everythingis in flux, and nothingis at rest.' Everythingis in
flux, even the beams,the timber,the buildingmaterialof which
the worldis made: earthand rocks,or a bronzecauldron-they
are all in flux. The beamsare rotting,the earthis washedaway
and blown away, the very rocks split and wither, the bronze
cauldronturnsinto greenpatina,or into verdigris:' all thingsare
in motion all the time, even though ... this escapesour senses',
as Aristotleexpressedit. Those who do not know and do not
think,believethat only the fuel is burned,whilethe bowlin which
it burnsremainsunchanged;for we do not see the bowl burning.
And yet, it burns: it may be eaten up by the fire it holds. We
do not see our childrengrow up, and change,and grow old, but
they do.
Thus thereare no solid bodies. Thingsare not reallythings,
they are processes,they are in flux. They are like fire, like a
flamewhich,thoughit may have a definiteshape,is a process,a
streamof matter,a river. All thingsare flames: fireis the very
building materialof our world; and the apparentstability of
thingsis merelydueto the laws,the measures,whichthe processes
in our worldare subjectto.
This, I believe,is Heraclitus'story; it is his ' message', the
'true word ' (the' logos'), to whichwe oughtto listen: ' Listening
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KARL R. POPPER
not to me but to the true account, it is wise to admit that all
things are one ': they are ' an everlastingfire, flaring up in
measures,and dyingdown in measures'.
I know very well that the traditional interpretationof
Heraclitus'philosophyhere restatedis not acceptedat present.
But the criticshave put nothingin its place-nothing, that is, of
philosophicalinterest. I shall brieflydiscusstheirnew interpretation in the next section. Here I wish only to stress that
Heraclitus'philosophy,by appealingto thought,to the word,to
argument,to reason,and by pointingout that we are living in a
world of things whose changes escape our senses, though we
knowthat they do change,createda new problem-the problem
of change. Thisproblemwas the moreurgentas his own account
of changewas difficultto understand. But this, I believe,is due
to the fact that he saw, more clearlythan his predecessors,the
problemthat was involvedin the veryidea of change.
For all changeis the changeof something:changepresupposes
somethingthat changes. And it presupposesthat, while changing, this somethingmust remainthe same. We may say that a
greenleaf changeswhen it turnsbrown; but we do not say that
the green leaf changeswhen we substitutefor it a brown leaf.
It is essentialto the idea of changethat the thing that changes
retains its identity while changing. And yet, it must become
somethingelse: it was green, and it becomes brown; it was
moist, and it becomesdry; it was hot, and it becomescold.
Thus everychangeis, in a way, the transitionof a thinginto
somethingwith opposite qualities(as Anaximanderand Anaximenes had seen). And yet, the changing thing must remain
identicalduringchange.
This is the problemof change. It led Heraclitusto a theory
which (partly anticipatingParmenides)distinguishesbetween
realityand appearance. ' The real natureof thingsloves to hide
itself. An unapparentharmonyis strongerthan the apparent
one.' Thingsare in appearance(and for us) oppositesto us, but
in truth (and for God) they are the same. ' Life and death,
being awake and being asleep, youth and old age, all these are
the same ... for the one turnedroundis the other and the other
turnedroundis the first. . . The path that leads up and the path
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B3ACKTO THE PRE-SOCRATICS
13
that leadsdownis the samepath ... Good and bad are identical
... For God, all thingsare beautifuland good andjust, but men
assumesome thingsto be unjust,and othersto be just . . . It is
not in the natureor characterof man to possesstrue knowledge,
thoughit is in the divinenature.'
Thus in truth(and for God) the oppositesare identical; it is
only to man that they appearas non-identical. And all things
are one-they are all part of the processof the world, the everlastingfire.
This theory of change appeals to the ' true word', to the
logos, to reason. Nothing is more real for Heraclitusthan
change. Yet his doctrine of the oneness of the world, of the
identityof opposites,and of appearanceand realitythreatenshis
doctrine of the reality of change.
For changeis the transitionfrom one oppositeto the other.
Thus if in truth the oppositesare identical,though they appear
different,then changeitself mightbe only apparent. If in truth,
and for God, all things are one, there might be, in truth, no
change.
This consequencewas drawnby Parmenides,the pupil (pace
Burnetand others)of the monotheistXenophaneswho said of
the one God: 'He always remainsin the same place, never
moving. It is not fitting for Him to go to differentplaces at
differenttimes. . . He is in no way similarto mortalmen, neither
in body nor in thought.'
Xenophanes'pupil, Parmenides,taught that the real world
was one, and that it always remainsin the same place, never
moving. It was in no way similarto what appearedto be to
mortalmen. The world was one, an undividedwhole, without
parts,homogeneous,and motionless: motion was impossiblein
such a world. In truth, there was no change. The world of
changewas an illustion.
Parmenidesbased this theory of an unchangingreality on
somethinglike a logical proof; a proof whichcan be presented
as proceedingfrom the single premiss ' What is not is not'.
From this, we canlderivethat the nothing-that whichis notdoes not exist; a result which Parmenidesinterpretsto mean
that the void does not exist. Thus the worldis full: it consists
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KARL R. POPPER
of one undividedblock, since any divisioninto partscould only
be due to separationof the parts by the void. (This is ' the
well-roundedtruth' which the goddessrevealedto Parmenides.)
In this full world,thereis no room for motion.
Only the delusivebelief in the realityof opposites-the belief
that not only what is exists but also what is not-leads to the
illusionof a world of change.
Parmenides'theorymaybe describedas the first hypotheticodeductivetheory of the world. The atomists took it as such;
and they assertedthat it was refutedby experience,sincemotion
does exist. Accepting the formal validity of Parmenides'
argument,they inferredfrom the falsity of his conclusion the
falsity of his premiss. But this meantthat the nothing-or the
void-existed. Consequently,therewas now no need to assume
that 'what is '-the full, that which fills some space-had no
parts; for its partscould now be separatedby the void. Thus
there are many parts whichareall ' full ': therearefull particles
in the world,separatedby emptyspace,andableto movein empty
space, each of thembeing 'full', undivided,indivisible,and unchanging. In this way the atomistsarrivedat a theoryof change
-a theory that dominatedscientificthought until 1900. It is
the theory that all change, and especially all qualitative change,
has to be explained by the spatial movement of unchangingbits of
matter.
The next great step in our cosmology was made when
Maxwell,inspiredby ideas of Faraday,replacedthis theoryby a
theoryof changingintensitiesof fields.
x
I havesketchedthe story,as I see it, of the Pre-Socratictheory
of change. I am of coursewell awareof the fact that my story
(which is based on Plato, Aristotle, and the doxographic
tradition)clashes at many points with the views of the experts,
Englishas well as German,andespeciallywiththe viewsexpressed
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15
by G. K. Kirk and J. E. Raven in their book, ThePre-Socratic
1957. I cannot,of course,examinetheirarguments
Philosophers,
in detail here, and especially not their minute exegesis of
variouspassagessome of whichare relevantto the differencesof
interpretation. (See, for example,Kirk and Raven'sdiscussion
of the question whetherthere is a referenceto Heraclitusin
Parmenides;cf. theirnote 1 on pp. 193f.,and note 1 on pp. 272.)
But I wish to say that I have examinedtheirargumentsand that
I have found many which seem to me unacceptable.
I will mention here only some points in connexion with
Heraclitus(althoughthereare otherpoints of equalimportance,
such as their commentson Parmenides).
The traditionalview, accordingto which Heraclitus'central
doctrinewasthatall thingsarein flux,wasattackedfortyyearsago
by Burnet. His main argument(discussedby me at length in
note 2 to Chapter2 of my OpenSociety)was that the theory of
changewas not new, and that only a new messagecould explain
the urgencywith which Heraclitusspeaks. This argumentis
repeatedby Kirkand Ravenwhentheywrite(pp. 186f.):' But all
Pre-Socraticthinkerswerestruckby the predominanceof change
in the worldof our experience.' About this attitudeI said in my
Open Society: ' Those who suggest . . . that the doctrine of
universal flux was not new . .. are, I feel, unconscious witnesses
to Heraclitus'originality,for they fail now, after2,400 years,to
grasp his main point.' In brief, they do not see the difference
betweenthe Milesianmessage: ' There is a fire in the house'
and Heraclitus'ssomewhatmore urgentmessage: 'The house is
on fire'. An implicit reply to this criticismcan be found on
p. 197 of the book by Kirk and Raven,wherethey write: 'Can
Heraclitusreallyhave thoughtthat a rock or a bronzecauldron,
for example, was invariablyundergoinginvisible changes of
material? Perhaps so; but nothing in the extant fragments
suggests that he did.' But is this so ? Heraclitus' extant
fragmentsabout the fire (Kirk and Raven, fragments220-222)
are interpretedby Kirk and Raven themselves as follows (p. 200):
'Fire is the archetypal form of matter.' Now I am not at all
sure what ' archetypal ' means here (especially in view of the fact
that we read a few lines later ' Cosmogony . .. is not to be found
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KARL R. POPPER
in Heraclitus'). But whatever'archetypal' may mean, it is
clear that once it is admittedthat Heraclitussays in the extant
fragmentsthat all matteris somehow(whetherarchetypicallyor
otherwise)fire,he also says that all matter,like fire,is a process;
which is preciselythe theory denied to Herac}itusby Kirk and
Raven.
Immediatelyaftersayingthat ' nothingin the extantfragments
suggests'that Heraclitusbelievedin continuousinvisiblechanges,
Kirk and Raven make the following methodologicalremark:
' It cannot be too stronglyemphasizedthat before Parmenides
and his apparentproof that the senseswerecompletelyfallacious
. .. gross departuresfrom commonsensemust only be accepted
whenthe evidencefor themis extremelystrong.' Thisis intended
to mean that the doctrinethat bodies of all materialsconstantly
undergo invisible changes representsa gross departurefrom
common sense, a departurewhich one ought not to expect in
Heraclitus.
IBut to quote Heraclitus: 'He who does not expect the
unexpectedwill not detectit: for him it will remainundetected,
and uncomprehended.' In fact, Kirk and Raven'slast argument is invalidon manygrounds. Long beforeParmenides,we
find ideas far removedfrom common sense in Anaximander,
Pythagoras,Xenophanes,and especiallyin Heraclitus. In fact,
the suggestionthat we should test the historicityof Heraclitus'
ideas-as we might indeed test those of Anaximenes-by standardsof ' commonsense'is a littlesurprising(whatever'common
sense' may mean here). For this suggestionruns counternot
only to Heraclitus' niotorious obscurity and oracular style,
confirmedby Kirkand Raven,but also to his burninginterestin
antinomyand paradox. And it runscounter,last but not least,
to the (in my view quiteabsurd)doctrinewhichKirk and Raven
finally attribute to Heraclitus (the italics are mine): '. . . that
also earthquakes
naturalchangesof all kinds[andthuspresumably
and great fires] are regular and balanced,and that the cause of this
balance is fire, the common constituent of things that was also
termedtheirLogos.' If this is Heraclitus'philosophy,then I
see no reasonto take any interestin it; at any rate, it is much
furtherremovedfromcommonsense(as I see it) thanthe inspired
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philosophywhich, in the name of common sense, is rejectedby
Kirk and Raven.
But the decisive point is, of course, that this inspired
philosophyis true,for all we know. Withhis uncannyintuition,
Heraclitussaw that things are processes,that our bodies are
flames,that ' a rock or a bronzecauldronwas invariablyundergoing invisiblechanges'. Kirk and Raven say (p. 197,note 1),
'Every time the fingerrubs, it rubs off an invisibleportion of
iron; yet whenit does not rub,whatreasonis thereto thinkthat
the iron is still changing?' The reason is that the wind rubs,
and that thereis alwayswind; or that iron turnsinvisiblyinto
rust-by oxidation,and this means,by slow burning; or that old
iron looks differentfrom new iron, just as an old man looks
differentfrom a child. This was Heraclitus'teaching,as the
extantfragmentsshow.
I suggest that Kirk and Raven's methodologicalprinciple
'that grossdeparturesfromcommonsensemustonly be accepted
when the evidencefor them is extremelystrong' might well be
replacedby the clearerand more importantprinciplethat gross
departures
ftom thehistoricaltraditionmustonlybe acceptedwhen
the evidencefor them is extremelystrong. This, in fact, is a
universalprincipleof historiography. Withoutit, historywould
be impossible. Yet it is constantlyviolatedby Kirkand Raven:
when,forexample,theytryto makePlato'sandAristotle'sevidence
suspect, with argumentswhich are partly circularand partly
(like the one from common sense)in contradictionto their own
story. And when they say that 'little seriousattemptseemsto
have been made by Plato and Aristotle to penetratehis [i.e.
Heraclitus']realmeaning' then I can onlysaythat the philosophy
outlinedby Plato and Aristotleseems to me a philosophythat
has real meaningand real depth. It is a philosophyworthyof a
greatphilosopher. Who, if not Heraclitus,was the greatthinker
who first realized that men are flames and that things are
processes? Are we really to believethat this great philosophy
was a ' post-Heracliteanexaggeration'(p. 197),and that it may
have been suggested to Plato, 'in particular, perhaps, by
Cratylus'? Who, I ask, was this unknown philosopherperhapsthe greatestand certainlythe boldestthinkeramongthe
Pre-Socratics
? Who was he, if not Heraclitus?
B
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KARL R. POPPER
XI
The earlyhistoryof Greekphilosophy,especiallythe history
from Thalesto Plato, is a splendidstory almosttoo good to be
true. In everygenerationwe find at least one new philosophy,
one new cosmology of staggeringoriginalityand depth. How
was this possible? Of course,one cannotexplainoriginalityand
genius. But one can try to throw some light on it. What was
the secretof the ancients? I suggestthat it was a tradition-the
traditionof criticaldiscussion.
I will try to put the problemmore sharply. In all or almost
all civilizations,we findsomethinglike religiousand cosmological
teaching,and in many societieswe find schools. Now schools,
especiallyprimitiveschools, have all, it appears,a characteristic
structure and function. Far from being places of critical
discussion,they make it their task to imparta definitedoctrine,
andto preserveit, pureand unchanged. It is the task of a school
to hand on the tradition,the doctrine of its founder, its first
master,to the nextgeneration,andto this end the mostimportant
thing is to keep the doctrineinviolate. A school of this kind
neveradmitsa new idea. New ideas are heresies,and they lead
to splits: should a member of the school try to change the
doctrine,then he is expelledas a heretic. But the hereticclaims,
as a rule, that his is the true doctrineof the founder. Thus not
even the inventoradmits that he has introducedan invention;
rather, he believes that he is returningto the true orthodoxy
which has somehowbeen perverted.
In this way, all changesof doctrine-if any-are surreptitious
changes. They are all presentedas re-statementsof the true sayings of the master,of his own words,his own meaning,his own
intentions.
It is clearthat, in a school of this kind, we cannot expectto
find a history of ideas, or even the materialfor s-ucha history.
For new ideas are not admitted to be new. Everythingis
ascribedto the master. All we mightreconstructis a historyof
schisms,and perhapsa historyof the defenceof certaindoctrines
againstthe heretics.
Therecannot,of course,be any rationaldiscussionin a school
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19
of this kind. There may be argumentsagainst dissentersand
heretics,or against some competingschools. But in the main,
it is with assertionand dogma and condemnationratherthan
argumentthat the doctrineis presented.
The greatexampleof a school of this kind amongthe Greek
philosophicschoolsis the ItalianSchoolfoundedby Pythagoras.
Comparedwith the Ionianschool, or with that of Elea,it had the
characterof a religiousorder,with a characteristicway of life,
and a secret doctrine. The story that a member,Hippasusof
Metapontum,was drownedat sea becausehe revealedthe secret
of the irrationalityof certain numbers,is characteristicof the
atmospheresurroundingthe Pythagoreanschool, whetheror not
thereis any truthin this story.
But amongGreekphilosophicschools,the earlyPythagoreans
were an exception. Leavingthem aside, we could say that the
characterof Greekphilosophy,and of the philosophicschools,is
strikingly different from the dogmatic type of school here
described. I have shown this by an example: the story of the
problemof change whichI have told is the story of a critical
debate,of a rationaldiscussion. New ideas are propoundedas
such, and arise as the result of open criticism. There are few,
if any, surreptitiouschanges. Instead of anonymitywe find a
historyof ideas and of their originators.
Hereis a uniquephenomenon,andit is closelyconnectedwith
the astonishingfreedomand creativenessof Greek philosophy.
How can we explainthis phenomenon? Clearly,whatwe have
to explainis the rise of a tradition. It is a traditionthat allows
or encouragescriticaldiscussionsbetweenvarious schools and,
more surprisinglystill, within one and the same school. For
nowhere outside the Pythagoreanschool do we find a school
devoted to the preservationof a doctrine. Instead, we find
changes,new ideas, modifications,and outrightcriticismof the
master.
(In Parmenideswe even find, at an earlydate, a most remarkable phenomenon-that of a philosopherwho propoundstwo
doctrines,one which he says is true, and one which he himself
describesas false: yet he makes the false doctrinenot simply
an object of condemnationor of criticism; ratherhe presentsit
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KARL R. POPPER
as the best possible account of the delusive opinion of mortal
men, and of the world of mere appearance-the best account
whicha mortalman can give.)
How and wherewas this criticaltraditionfounded? This is
a problem deservingserious thought. This much is certain:
Xenophaneswho broughtthe Ionian traditionto Elea was fully
consciousof the fact thathis own teachingwaspurelyconjectural,
and that othersmight come who knew better. I shall refer to
this point againin my next andlast section.
If we try to search for the first signs of this new critical
attitude,this new freedomof thought,then we are led back to
Anaximander'scriticismof Thales. Hereis a most strikingfact.
Anaximandercriticizeshis masterand kinsman,one of the Seven
Sages,the founderof the Ionian school. He was, accordingto
tradition,only aboutfourteenyearsyoungerthan Thales,and he
must have developedhis criticismand his new ideas while his
masterwas alive. (They seem to have died within a few years
of each other.) But thereis no trace,in the sources,of a story
of dissent, of any quarrel,or of any schism.
This suggests,I think, that it was Thales who founded the
new traditionof freedom-based upon a new relationbetween
masterand pupil-and who thus createda new type of school,
utterly differentfrom the Pythagoreanschool. He seems to
have been able to tolerate criticism. And what is more, he
seems to have created the traditionthat one ought to tolerate
criticism.
Yet I like to think that he did even more than this. I can
hardly imagine a master-pupilrelation in which the master
merely tolerates criticism without actively encouragingit. It
does not seem to me possiblethat a pupil who is beingtaughtin
the dogmatic attitude would ever dare to criticizethe dogma
(least of all that of a famous sage), and to vent his criticism. It
seemsto me an easierand simplerexplanationto assumethat the
master encourageda critical attitude-possibly not from the
outset, but only after he was struck by the pertinenceof some
questionsasked by the pupil without any criticalintention.
However this may be, the conjecturethat Thales actively
encouragedcriticismin his pupilswould explainthe fact that the
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criticalattitudetowardsthe master'sdoctrinebecamepart of the
Ionian school-tradition. I like to think that Thales was the
first teacherwho said to his pupils: 'This is how I see thingshow I believethatthingsare. Tryto improveuponmy teaching!'
(Those who believe that it is ' unhistorical' to attributethis
undogmaticattitudeto Thalesmay againbe remindedof the fact
that, only two generationslater, we find a similar attitude
consciously and clearly formulated in the fragments of
Xenophanes.) At any rate, there is the historicalfact that the
Ionianschoolwas the firstin whichpupilscriticizedtheirmasters,
in one generationafterthe other. Therecan be little doubt that
the Greektraditionof philosophicalcriticismhad its main source
in Ionia.
It was a momentousinnovation. It meant a break with the
dogmatictraditionwhich permitsonly one school doctrine,and
the introductionin its place of a traditionthat admitsa plurality
of doctrineswhich all try to approachthe truth, by means of
criticaldiscussion.
It thus leads, almost by necessity,to the realizationthat our
attemptsto see, and to find, the truth,are not final, but open to
improvement;that our knowledge,our doctrine,is conjectural;
that it consistsof guesses,of hypotheses,ratherthan of finaland
certaintruths; and that criticismand criticaldiscussionare our
only means of gettingnearerto the truth. It thus leads to the
traditionof bold conjecturesand of free criticism,the tradition
which createdthe rationalor scientificattitude,and with it our
Westerncivilization,the only civilizationwhich is based upon
science.
In this rationalisttradition,bold changesof doctrineare not
forbidden. Rather, innovation is encouraged,is regardedas
success,as improvement,if it is based on the resultof a critical
discussion of its predecessors. And the very boldness of an
innovationis admired; for it can be controlledby the severityof
its critical examination. Thus changes of doctrine, far from
being made surreptitiously,are traditionally handed down
togetherwith the olderdoctrinesand the namesof the innovator.
Thusthe material,at least, for a historyof ideas becomespartof
the school tradition.
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KARL R. POPPER
To my knowledge,the critical or rationalisttradition was
invented only once. It was lost after two or three centuries,
perhapsowingto the rise of the Aristoteliandoctrineof episteme,
of certain and demonstrableknowledge. It was re-discovered
and consciously revived by the Renaissance, especially by
Galileo Galilei.
XII
I now come to my last and most centralcontention. It is
this. Therationalisttradition,the traditionof criticaldiscussion,
representsthe only practicableway of expandingour knowledge
-conjectural or hypotheticalknowledge,of course. There is
no otherway. More especially,thereis no way that startsfrom
observation or experiment. In the developmentof science,
observationsand experimentsplay only the role of critical
arguments. And they play this rOle alongside other,
non-observationalarguments. It is an importantrole; but the
significanceof observationsand experimentsdepends entirely
upon the questionwhetheror not they may be used to criticize
theories.
Accordingto the theoryof knowledgehereoutlined,thereare
in the main only two ways in whichtheoriesmay be superiorto
others: they may explainmore, and they may be bettertestedthat is, they may be more fully and more criticallydiscussed,in
the light of all we know, of all the objectionswe can think of,
and especiallyalso in the light of observationalor experimental
tests which were designed with the aim of criticizing the
theory.
There is only one element of rationalityin our attemptsto
know the world: it is the criticalexaminationof our theories.
These theoriesthemselvesare guesswork. We do not know, we
only guess. If you ask me 'How do you know' my replywould
be 'I don't: I only proposea guess. If you areinterestedin my
problem,I shall be most happyif you criticizemy guess, and if
you offer your counter-proposalswhich I, in turn, will try to
criticize.'
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23
This, I believe,is the truetheoryof knowledge( whichI wish
to submitfor your criticism):the true descriptionof a practice
whicharosein Ioniaandwhichis incorporatedin modernscience,
thoughthereare manyscientistswho still believein the Baconian
myth of induction.
Two of the greatestwho clearlysaw that there was no such
thingas an inductiveprocedure,andwho clearlyunderstoodwhat
I regard as the true theory of knowledge, were Galileo and
Einstein. Yet the ancientsalso knewit. Incredibleas it sounds,
we find a clear recognitionand formulationof this theory of
rational knowledge almost immediatelyafter the practice of
critical discussionhad begun. Our oldest extant fragmentsin
this field are those of Xenophanes. I will presentthem here in
an orderthat suggeststhat it was the boldnessof his attack and
the gravity of his problemswhich made him conscious of the
fact that all our knowledgewas guessworkand that, notwithstandingthis fact, we may, by searchingfor the better,find it in
the course of time. Here are four of the fragments from
Xenophanes'writings.
But if cattle or horses or lions had hands and could draw
And sculpturelike men, then the horses would draw their gods
Like horses, and cattle like cattle, and all would then shape
Bodies of gods in the likeness,each kind, of its own.
The gods did not reveal, from the beginning,
All things to us; but in the course of time,
By seeking for it, men find out the better.
Let us conjecturethat this is like truth.
But as for certaintruth, no man has known it,
Nor will he know it; neither of the gods
Nor yet of all the things of which I speak.
And if by any chance he were to utter
Finality, he would himself not know it:
For all is but a woven web of guesses.
To show that Xenophaneswas not alone, I may also repeat
here two of Heraclitus'sayingswhich I have quoted beforein a
different context. Both express the conjecturalcharacterof
humanknowledge,and the secondrefersto its daring,to the need
to anticipateboldly what we do not know.
It is not in the natureor characterof man to possesstrueknowledge,
though it is in the divine nature . . . He who does not expect
the unexpectedwill not detectit: for him it will remainundetected,
and uncomprehended.
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KARL R. POPPER
My last quotation is a very famous one from Democritus:
Nothing do we know from having seen it; for the truth is hidden
in the deep.
This is how the critical attitude of the Pre-Socratics prepared
and made possible the ethical rationalism of Socrates. I am
alluding to his belief that the search for truth through critical
discussion was a way of life-in fact, the best he knew.
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