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The pluralism of Greek mathematics G E R Lloyd

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The pluralism of Greek mathematics G E R Lloyd
8
The pluralism of Greek ‘mathematics’
G . E . R. Ll oy d
Greek mathēmatikē, as has often been pointed out, is far from being an
exact equivalent to our term ‘mathematics’. The noun mathēma comes from
the verb manthanein that has the entirely general meaning of ‘to learn’.
A mathēma can then be any branch of learning, or anything learnt, as when
in Herodotus (1 207) Croesus refers to the mathēmata – what he has learnt –
from his own bitter experiences. So the mathēmatikos is, strictly speaking, the person who is fond of learning in general, as indeed it is used in
Plato’s Timaeus at 88c where the point at issue is the need to strike a balance
between the cultivation of the intellect and that of the body, the principle
that later became encapsulated in the dictum ‘mens sana in corpore sano’.
Yet Plato also recognizes certain special branches of the mathēmata, as
when in the Laws at 817e the Athenian Stranger speaks of those that are
appropriate for free citizens as those that relate to numbers, to the measurement of lengths, breadths and depths, and to the study of the stars, in other
words, very roughly, arithmetic, geometry and astronomy. In Hellenistic
Greek mathēmatikos is used more often of the student of the heavens in
particular (whether what we should call the astronomer or the astrologer)
than of the mathematician in general in our sense.
Whether we should think of either what we call mathematics or what
we call philosophy as well-defined disciplines before Plato is doubtful.
I have previously discussed the problems so far as philosophy is concerned.1 Those whom modern scholars conventionally group together as
‘the Presocratic philosophers’ are a highly heterogeneous set of individuals,
most of whom would not have recognized most of the others as engaged in
the same inquiry as themselves. Their interests spanned in some, but not all,
cases what we call natural philosophy (the inquiry into nature), cosmology,
ontology, epistemology, philosophy of language and ethics, but the ways
in which those interests were distributed among the different individuals
concerned varied considerably.
It is true that we have one good fifth-century bce example of a thinker
most of whose work (to judge from the very limited information we have
294
1
Lloyd 2006b.
The pluralism of Greek ‘mathematics’
about that) related to, or used, one or other branch of mathematics, namely
Hippocrates of Chios. He was responsible not just for important particular
geometrical studies, on the quadrature of lunules, but also, maybe, for a first
attempt at systematizing geometrical knowledge, though whether he can
be credited with a book entitled (like Euclid’s) Elements is more doubtful.
Furthermore in his other investigations, such as his account of comets,
reported by Aristotle in the Meteorology, he used geometrical arguments to
explain the comet’s tail as a reflection.
Yet most of those to whom both ancient and modern histories of preEuclidean Greek mathematics devote most attention were far from just
‘mathematicians’ in either the Greek or the English sense. Philolaus,
Archytas, Democritus and Eudoxus all made notable contributions to one
or other branches of mathēmatikē, but all also had developed interests in
one or more of the studies we should call epistemology, physics, cosmology and ethics. A similar diversity of interests is also present in what we are
told of the work of such more shadowy figures as Thales or Pythagoras. The
evidence for Thales’ geometrical theorems is doubtful, but Aristotle (who
underlines the limitations of his own knowledge about Thales) treats him as
interested in what he, Aristotle, termed the material cause of things, as well
as in soul or life. Pythagoras’ own involvement in geometry and in harmonics has again been contested,2 and the more reliably attested of his interests
relate to the organization of entities in opposite pairs, and, again, to soul.
These remarks have a bearing on the controversy on the question of
whether deductive argument, in Greece, originated in ‘philosophy’ and
was then exported to ‘mathematics’,3 or whether within mathematics it was
an original development internal to that discipline.4 Clearly when neither
‘philosophy’ nor ‘mathematics’ were well-defined disciplines, it is hard to
resolve that issue in the terms in which it was originally posed, although, to
be sure, the question remains as to whether the Eleatic use of reductio arguments did or did not influence the deployment of arguments of a similar
type by such figures as Eudoxus.
If we consider the evidence for the investigation of what Knorr, in other
studies,5 called the three ‘traditional’ mathematical problems, of squaring
the circle, the duplication of the cube and the trisection of an angle, those
who figure in our sources exhibit very varied profiles. Among the ten or so
individuals who are said to have tackled the problem of squaring the circle
2
3
4
5
Burkert 1972.
Szabó 1978.
Knorr 1981.
Knorr 1986.
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it is clear that ideas about what counts as a good, or even a proper, method
of doing so differed.6 At Physics 185a16–17 Aristotle distinguishes between
fallacious quadratures that are the business of the geometer to refute, and
those where that is not the case. In the former category comes a quadrature
‘by way of segments’ which the commentators interpret as lunules and
forthwith associate with the most famous investigator of lunules, whom
I have already mentioned, namely Hippocrates of Chios. Yet even though
there is another text in Aristotle that accuses Hippocrates of some mistake
in quadratures (On Sophistical Refutations 171b14–16), it may be doubted
whether Hippocrates committed any fallacy in this area.7 In the detailed
account that Simplicius gives us of his successful quadrature of four specific
types of lunules, the reasoning is throughout impeccable. Quite what fallacy
Aristotle detected then remains somewhat of a mystery.
But two other attempts are also referred to by Aristotle and dismissed
either as ‘sophistic’ or as not the job of the geometer to disprove. Bryson
is named at On Sophistical Refutations 171b16–18 as having produced an
argument that falls in the former category: according to the commentators,
it appealed to a principle about what could be counted as equals that was
quite general, and thus far it would fit Aristotle’s criticism that the reasoning
was not proper to the subject-matter.
Antiphon’s quadrature by contrast is said not to be for the geometer to
refute (Physics 185a16–17) on the grounds that it breached the geometrical principle of infinite divisibility. It appears that Antiphon proceeded by
inscribing increasingly many-sided regular polygons in a circle until – so
he claimed – the polygon coincided with the circle (which had then been
squared). The particular interest of this procedure lies in its obvious similarity to the so-called but misnamed method of exhaustion introduced by
Eudoxus in the fourth century. This too uses inscribed polygons and claims
that the difference between the polygon and the circle can be made as small
as one likes. It precisely does not exhaust the circle. If Antiphon did indeed
claim that after a finite number of steps the polygon coincided with the
circle, then that indeed breached the continuum assumption. But of course
later mathematicians were to claim that the circle could nevertheless be
treated as identical with the infinitely-sided inscribed rectilinear figure.
Other solutions were proposed by other figures, by a certain Hippias for
instance and by Dinostratus. Whether the Hippias in question is the famous
sophist of that name has been doubted, precisely on the grounds that the
6
7
Mueller 1982 gives a measured account.
Lloyd 2006a reviews the question.
The pluralism of Greek ‘mathematics’
device attributed to him, the so-called quadratrix, is too sophisticated for
the fifth century.
Although much remains obscure about the precise claims made in different attempts at quadrature, it is abundantly clear first that different investigators adopted different assumptions about the legitimacy of different
methods, and second that those investigators were a heterogeneous group.
Some were not otherwise engaged in mathematical studies at all, at least to
judge from the evidence available to us. An allusion in Aristophanes (Birds
1001–5) suggests that the topic of squaring the circle had by the end of the
fifth century become a matter of general interest, or at least the possible
subject of anti-intellectual jokes in comedy.
Among those I have mentioned in relation to quadratures several are
generally labelled ‘sophists’, this too a notoriously indeterminate category
and one that evidently cannot be seen as an alternative to ‘mathematician’.
As is well known Plato does not always use the term pejoratively, even
though he certainly has severe criticisms to offer, both intellectual and
moral, of several of the principal figures he calls ‘sophists’. Yet Plato himself
provides plenty of evidence of the range of interests, both mathematical
and non-mathematical, of some of those he names as such. As regards the
Hippias he calls a sophist, those interests included astronomy, geometry,
arithmetic, but also, for instance, linguistics: however, whether the music
he also taught related to the mathematical analysis of harmonics or was
a matter of the more general aesthetic evaluation of different modes is
unclear. Again, the fragments that are extant from Antiphon’s treatise Truth
deal with questions in cosmology, meteorology, geology and biology.8
Protagoras, who is said by Plato to have been the first to have taught for a
fee, famously claimed, according to Aristotle Metaphysics 998a2–4, that the
tangent does not touch the circle at a point, a meta-mathematical objection
that he raised against the geometers.
Thus far I have suggested some of the variety within what the Greeks
themselves thought of as encompassed by mathēmatikē together with
some of the heterogeneity of those who were described as engaged in
‘mathematical’ inquiries. But in view of some persistent stereotypes of
Greek mathematics it is important to underline the further fundamental
disagreements (1) about the classification of the mathematical sciences and
the hierarchy within them, (2) about the question of their usefulness, and
8
The identification of the author of this treatise with the Antiphon whose quadrature is
criticized by Aristotle is less disputed than the question of whether the sophist is identical with
the author called Antiphon whose Tetralogies are extant.
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especially (3) on what counts as proper, valid, arguments and methods. Let
me deal briefly with the first two questions before exemplifying the third a
little more fully.
(1) Already in the late fifth and early fourth centuries bce a divergence
of opinion is reported as between Philolaus and Archytas. According to
Plutarch (Table Talk 8 2 1, 718e) Philolaus insisted that geometry is the
primary mathematical study (its ‘metropolis’). But Archytas privileged
arithmetic under the rubric of logistikē (reckoning, calculation, Fr. 4). The
point is not trivial, since how precisely geometry and arithmetic could
be considered to form a unity was problematic. According to the normal
Greek conception, ‘number’ is defined as an integer greater than 1. In this
view, arithmetic dealt with discrete entities. But geometry treated of an
infinitely divisible continuum. Nevertheless both were regularly included
as branches of ‘mathematics’, sister branches, indeed, as Archytas called
them (Fr. 1). The question of the status of other studies was more contested. For Aristotle, who had, as we shall see, a distinctive philosophy
of mathematics, such disciplines as optics, harmonics and astronomy
were ‘the more physical of the mathēmata’ (Physics 194a7–8). The issue of
‘mechanics’ was particularly controversial. According to the view of Hero,
as reported by Pappus (Collection Book 8 1–2), mechanics had two parts,
the theoretical which consisted of geometry, arithmetic, astronomy and
physics, and the practical that dealt with such matters as the construction
of pulleys, war machines and the like. However, a somewhat different view
was propounded by Proclus (Commentary on Euclid’s Elements 41.3 – 42.8)
when he included what we should call statics, as well as pneumatics, under
‘mechanics’.
(2) That takes me to my next topic, the issue of the usefulness of mathematics, howsoever construed. Already in the classical period there was a
clear division between those who sought to argue that mathematics should
be studied for its practical utility, and those who saw it rather as an intellectual, theoretical discipline. In Xenophon’s Memorabilia 4 7 2–5 Socrates
is made to insist that geometry is useful for land measurement, astronomy
for calendar regulation and navigation, and so on, and he there dismissed
the more theoretical or abstract aspects of those subjects. Similarly Isocrates
too distinguished the practical and the theoretical sides of mathematical
studies and in certain circumstances favoured the former (11 22–3, 12 26–8,
15 261–5). Yet Plato of course took precisely the opposite view. It is not for
practical, mundane, reasons that mathematics is worth studying, but rather
as a training for the soul in abstract thought. But even some who emphasized practical utility sometimes defined that very broadly. It is striking that
The pluralism of Greek ‘mathematics’
in the passage just quoted from Pappus he included both the construction
of models of planetary motion and that of the marvellous gadgets of the
‘wonder-workers’ among ‘the most necessary of the mechanical arts from the
point of view of the needs of life’. Meanwhile the most ambitious claims for
the all-encompassing importance of ‘mathematics’ were made by the neoPythagorean Iamblichus at the turn of the third and fourth centuries ce. He
argued in On the Common Mathematical Science (ch. 32: 93.11–94.21) that
mathematics was the source of understanding in every mode of knowledge,
including in the study of nature and of change.
(3) From among the many examples that illustrate how the question of
the proper method in mathematics was disputed let me select just five.
(3.1) In a famous and influential passage in his Life of Marcellus
(ch. 14, cf. Table Talk 8 2 1, 718ef) Plutarch interprets Plato as having
banned mechanical methods from geometry on the grounds that these
corrupted and destroyed the pure excellence of that subject, and it is true
that Plato had protested that to treat mathematical objects as subject to
movement was absurd. The first to introduce such degenerate methods,
according to Plutarch, were Eudoxus and Archytas. Indeed we know from
a report in Eutocius (Commentary on Archimedes Sphere and Cylinder 2, 3
84.12–88.2) that Archytas solved the problem of finding two mean proportionals on which the duplication of a cube depended by means of a complex
three-dimensional kinematic construction involving the intersection of
three surfaces of revolution, a right cone, a cylinder and a tore. Plutarch
even goes on to suggest that Archimedes himself agreed with the Platonic
view (as Plutarch represents it) that the work of an ‘engineer’ was ignoble
and vulgar. Most scholars are agreed first that that most probably misrepresents Archimedes, and secondly that few practising mathematicians would
have shared Plutarch’s expressed opinion as to the illegitimacy of mechanical methods in geometry.
(3.2) My second example comes from Archimedes himself and concerns
precisely how he endorsed the usefulness of mechanics, as a method of
discovery at least. In his Method (2 428.18–430.18) he sets out what he
describes as his ‘mechanical’ method which depends first on an assumption of indivisibles and then on imagining geometrical figures as balanced
against one another about a fulcrum. The method is then applied to get
the area of a segment of a parabola, but while Archimedes accepts the
method as a method of discovery, he puts it that the results have thereafter
to be demonstrated rigorously using the method of exhaustion standard
throughout Greek geometry. At the same time the method is useful ‘even
for the proofs of the theorems themselves’ in a way he explains (Method
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428.29–430.1): ‘it is of course easier, when we have previously acquired, by
the method, some knowledge of the questions, to supply the proof, than it
is to find it without any previous knowledge’. We should note that what is at
stake is not just the question of admissible methods, but that of what counts
as a proper demonstration.
(3.3) For my third example I turn to Hero of Alexandria.9 Although he
frequently refers to Archimedes as if he provided a model for demonstration, his own procedures sharply diverge, on occasion, from his. In the
Metrica, for instance, he sometimes gives an arithmetized demonstration
of geometrical propositions, that is he includes concrete numbers in his
exposition. Moreover in the Pneumatica especially he allows exhibiting a
result to count as a proof. Thus at 1 16.16–26 and at 26.25–28 he gives what
we would call an empirical demonstration of propositions in pneumatics,
expressing his own clear preference for such by contrast with the merely
plausible reasoning used by the more theoretically inclined investigators.
In both respects his procedures breach the rules laid down by Aristotle in
the Posterior Analytics, both in that he permits ‘perceptible’ proofs and does
not base his arguments on indemonstrable starting points and in that he
moves from one genus of mathematics to another. If we think of precedents
for his procedures, then they have more in common with the suggestion
that Socrates makes to the slave-boy in Plato’s Meno (84a), namely that if
he cannot give an account of the solution to the problem of doubling the
square, he can point to the relevant line.
(3.4) Fourthly there is Ptolemy’s redeployment of the old dichotomy
between demonstration and conjecture in two contexts in the opening
books of the Syntaxis and of the Tetrabiblos. In the former (Syntaxis 1 1,
1 6.11–7.4) he discusses the difference between mathēmatikē, ‘physics’ and
‘theology’. The last two studies are conjectural, ‘physics’ because of the instability of what it deals with, ‘theology’ because of the obscurity of its subject.
Mathēmatikē, by contrast, which here certainly includes the mathematical astronomy that he is about to expound in the Syntaxis, alone of these
three is demonstrative, since it is based on the incontrovertible methods
of geometry and arithmetic. Whatever we may think about the difficulties
that Ptolemy himself registers, in practice, in living up to this ideal when
it comes, for instance, to his account of the movements of the planets
in latitude, it is clear what his ideal is. Moreover when in the Tetrabiblos
(1 1, 3.5–25, 1 2, 8.1–20) he speaks of the other branch of the study of the
heavens, that which engages not in the prediction of the movements of the
9
Cf. Tybjerg 2000: ch. 3.
The pluralism of Greek ‘mathematics’
heavenly bodies, but in that of events on earth on their basis – astrology,
in other words, on our terms – that study is downgraded precisely on the
grounds that it cannot deliver demonstration. It is conjectural, though he
would claim that it is based on tried and tested assumptions.
(3.5) Fifthly and finally there are Pappus’ critical remarks, in the opening
chapters (1–23) of Book 3 of his Collection, on certain procedures based on
approximations that had been used in tackling the problem of finding two
mean proportionals in order to solve the Delian problem, of doubling the
cube.10 Although certain stepwise approximations can yield a result that is
correct, they fall short, in Pappus’ view, in rigour. Pappus himself distinguishes between planar, solid and linear problems in geometry and insists that
each has its own procedures appropriate for the subject matter in question.
What we find in all of the cases I have taken is a sensitivity not just to the
correctness of results or the truth of conclusions, but to the appropriateness
or otherwise of the methods used to obtain them. It is not enough just to
know the truth of a theorem: nor is it enough to have some means of justifying the claim to such knowledge. No: what is required is that the method of
justification be the correct one for the field of inquiry concerned according
to the particular standards of correctness of the author in question. That is
the recurrent demand: yet it is clearly not the case that all Greek investigators who would have considered themselves mathēmatikoi agreed on what
is appropriate in each type of case or had uniform views on what counts as
a demonstration.
Similar second-order disputes recur in most other areas of inquiry that
the Greeks engaged in, and this too is worth illustrating since it suggests
that the phenomenon we have described in mathematics is symptomatic of
more general tendencies in Greek thought. Sometimes we find such disagreements within what is broadly the same discipline, sometimes across different disciplines. In medicine the Hippocratic treatise On Ancient Medicine
provides examples of both kinds. The author first castigates other doctors
who try to base medical practice on what he calls ‘hypotheses’, arbitrary
postulates such as ‘the hot’, ‘the cold’, ‘the wet’, ‘the dry’ and anything else
they fancy (CMG 1 1, 36.2–21). In this author’s view, that is wrong-headed
since medicine is and has long been based on experience. The investigation
of what happens under the earth or in the sky may be forced to rely on such
postulates, but they are a disaster in medicine, where they have the result of
narrowing down the causal principles of diseases. While that drives a wedge
between medicine and ‘meteorology’, he goes on in chapter 20 (51.6–18)
10
I may refer to the detailed analysis in Cuomo 2000: ch. 4.
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specifically to attack the importation into medicine of methods and ideas
that he associates with ‘philosophy’, by which he here means speculative
theories about such topics as the constitution of the human body. For good
measure he insists that if one were to engage in that study, the proper way
of doing so would be to start from medicine.
Medicine provides particularly striking examples of second-order
debates parallel to those in mathematics: indeed in the Hellenistic period
the disagreements among the medical sects were as much about methods
and epistemology as they were about medical practice. But other fields too
exhibit similar fundamental divisions between competing approaches. In
music theory, Barker has explored the analogous disputes first between
practitioners on the one hand, and theoretical analysts on the other, and
then, among the latter, between those who treated musical sound in geometrical terms, as an infinitely divisible continuum, and those who adopted
an analysis based rather on arithmetic.11 Further afield I may simply remark
that the methods and aims of historiography are the subject of explicit
comment from Herodotus onwards. His views were criticized, implicitly,
by his immediate successor Thucydides, who contrasts history as entertainment with his own ambition to provide what he calls a ‘memorial for
eternity’ (1 21). But to achieve that end depended, of course, on the critical
evaluation of eyewitness accounts, as well as an assumption that certain
patterns of behaviour repeat themselves thanks to the constancy of human
nature.
With the development of both the practice and the teaching of rhetoric –
the art of public speaking – goes a new sense of what it takes to persuade an
audience of the strength of your case – and of the weakness of your rivals’
position. Both the orators and the statesmen deployed a rich vocabulary of
terms, such as apodeiknumi, epideiknumi and cognates, to express the claim
that they have proved their point, as to the facts of the matter in question, as
to the guilt or innocence of the parties concerned, or as to the benefits that
would accrue from the policies they advocated.
Yet that very same vocabulary was taken over first by Plato and then
by Aristotle to contrast what they claimed to be strict demonstrations on
the one hand with the arguments that they now downgraded as merely
plausible or persuasive, such as were used in the law courts and political
assemblies – and this takes us back to mathematics, since it provides the
essential background to the claims that some, but not all, mathematicians
made about the strictest mode of demonstration that they could deliver.
11
Barker 1989, 2000.
The pluralism of Greek ‘mathematics’
Aristotle was, of course, the first to propose an explicit definition of rigorous demonstration, which must proceed by way of valid deductive argument from premisses that are not just true, but also necessary, primary,
immediate, better known than, prior to and explanatory of the conclusions.
Furthermore Aristotle draws up a more elaborate taxonomy of arguments
than Plato had done, distinguishing demonstrative, dialectical, rhetorical,
sophistic and eristic reasoning according first to the aims of the reasoner
(which might be the truth, or victory, or reputation) and secondly to the
nature of the premisses used (necessary, probable, or indeed contentious).
Yet while the ideal that Aristotle sets for philosophy and for mathematics
is rigorous, axiomatic–deductive, demonstration, he not only allows that
the rhetorician will rely on what he calls rhetorical demonstration, but
concedes that in philosophy itself there may be stricter and looser modes,
appropriate to different subject matter.12
The goal the philosophers set themselves was certainty – where the conclusions reached were, supposedly, immune to the types of challenges that
always occurred in the law courts and assemblies. Yet from some points
of view the best area to exemplify this was not philosophy itself (ontology,
epistemology or ethics) but, of course, mathematics. However, the attitudes
of both Plato and Aristotle themselves towards mathematics were distinctly
ambivalent – not that they agreed on the status of that study. For Plato,
the inquiries the mathematician engages in are inferior to dialectic itself:
they are part of the prior training for the philosopher, but do not belong to
philosophy itself. The grounds for this that he puts forward in the Republic
are twofold, that the mathematician uses diagrams and that he takes his
‘hypotheses’ for granted, as ‘clear to all’.13 So although mathematics studies
intelligible objects and so is superior to any study devoted to perceptible
ones, it is inferior to dialectic which is purportedly based ultimately on an
‘unhypothesised starting point’, the idea of the Good.
Aristotle, by contrast, clearly accepts that mathematical arguments can
meet the requirements of the strictest mode of demonstration, since he
privileges mathematical examples to illustrate that mode in the Posterior
12
13
Lloyd 1996: ch. 1.
The interpretation of the expression ‘as clear to all’, hōs panti phanerōn, in the Republic
510d1, is disputed. My own view is that Plato is unlikely not to have been aware that many
of the hypotheses adopted by the mathematicians were contested (including for example the
definitions of straight line and point). When Socrates says that the mathematicians give no
account to themselves or anyone else about their starting-points, it would seem that this is
their claim, rather than (as it has generally been taken) their warrant. Burnyeat (2000: 37),
however, has argued that there is no criticism of mathematics in this text, but simply an
observation of an inevitable feature of their methods.
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Analytics. But mathematics suffers from a different shortcoming, in his
view, which relates to the ontological status of the subject matter it deals
with. Unlike Plato, who suggested that mathematics studies separate intelligible objects that are intermediate between the Forms and sensible particulars, Aristotle argued that mathematics is concerned with the mathematical
properties of physical objects.14 While physical objects meet the requirements of substance-hood, what mathematics studies belongs rather to the
category of quantity than to that of substance.
While Plato and Aristotle disagreed about the highest mode of philosophizing, ‘dialectic’ in Plato’s case, ‘first philosophy’ in Aristotle’s, they
both considered philosophy to be supreme and mathematics to be subordinate to it. Yet mathematics obviously delivered demonstrations, and
exemplified the goal of the certainty and incontrovertibility of arguments,
far more effectively than metaphysics, let alone than ethics. Once Euclid’s
Elements had shown how virtually the whole of mathematical knowledge
could be represented as a single, comprehensive system, derived from a
limited number of indemonstrable starting points, that model exerted very
considerable influence as an ideal, not just within the mathematical disciplines, but well beyond them.15 Euclid’s own Optics, like many treatises in
harmonics, statics and astronomy, proceeded on an axiomatic–deductive
basis, even though the actual axioms Euclid invoked in that work are problematic.16 More remarkably Galen sought to turn parts of medicine into an
axiomatic–deductive system just as Proclus did for theology in his Elements
of Theology.17 The prestige of proof ‘in the geometrical manner’, more
geometrico, made it the ideal for many investigations despite the apparent
difficulties of implementing it.
The chief problem lay not with deductive argument itself, but with its
premisses. Aristotle had shown that strict demonstration must proceed
14
15
16
17
Lear 1982.
As noted, the question of whether Hippocrates of Chios had a clear notion of ultimate
starting-points or axioms in his geometrical studies is disputed. In his quadratures of
lunes he takes a starting-point that is itself proved, and so not a primary premiss. Ancient
historians of mathematics mention the contributions of Archytas, Eudoxus, Theodorus and
Theaetetus leading up to Euclid’s own Elements, but while the commentators on that work
identify particular results as having been anticipated by those and other mathematicians, the
issue of how systematic their overall presentation of mathematical knowledge was remains
problematic.
Thus one of Euclid’s definitions in the Optics (def. 3, 2.7–9: cf. Proposition 1, 2.21–4.8) states
that those things are seen on which visual rays fall, while those are not seen on which they do
not. That seems to suggest that visual rays are not dense, a conception that conflicts with the
assumption of the infinite divisibility of the geometrical continuum. See Brownson 1981; Smith
1981; Jones 1994.
Lloyd 2006c.
The pluralism of Greek ‘mathematics’
from premisses that are themselves indemonstrable – to avoid the twin
flaws of circular argument and an infinite regress. If the premisses could
be proved, then they should be, and that in turn meant that they could
not be considered ultimate, or primary, premisses. The latter had to be
self-evident, autopista, or ex heautōn pista. Yet the actual premisses we find
used in different investigations are very varied. To start with, the kinds
or categories of starting points needed were the subject of considerable
terminological instability. Aristotle distinguished three types, definitions,
hypotheses and axioms, the latter being subdivided into those specific to
a particular study, such as the equality axiom, and general principles that
had to be presupposed for intelligible communication, such as the laws of
non-contradiction and excluded middle. Euclid’s triad consisted of definitions, common opinions (including the equality axiom) and postulates.
Archimedes in turn begins his inquiries into statics and hydrostatics by
setting out, for example, the postulates, aitēmata, and the propositions that
are to be granted, lambanomena, and elsewhere the primary premisses are
just called starting points or principles, archai.
As regards the actual principles that figure in different investigations, they
were far from confined to what Aristotle or Euclid would have accepted as
axioms. In Aristarchus’ exploration of the heliocentric hypothesis, he set out
among his premisses that the fixed stars and the sun remain unmoved and
that the earth is borne round the sun on a circle, where that circle bears the
same proportion to the distance of the fixed stars as the centre of a sphere to
its surface. Archimedes, who reports those hypotheses in the Sand-Reckoner
2 218.7–31, remarks that strictly speaking that would place the fixed stars
at infinite distance. The assumption involves, then, what we would call an
idealization, where the error introduced can be discounted. But in his only
extant treatise, On the Sizes and Distances of the Sun and Moon, Aristarchus’
assumptions include a value for the angular diameter of the moon as 2°, a
figure that is far more likely, in my view, to have been hypothetical in the
sense of adopted purely for the sake of argument, than axiomatic in the
sense of accepted as true. Meanwhile outside mathematics, we find Galen,
for example, taking the principles that nature does nothing in vain, and
that nothing happens without a cause, as indemonstrable starting points
for certain deductions in medicine. In Proclus, the physical principles that
natural motion is from, to, or around the centre, are similarly treated as
indemonstrable truths on which natural philosophy can be based.
The disputable character of many of the principles adopted as axiomatic
is clear. Euclid’s own parallel postulate was attacked on the grounds that
it should be a theorem proved within the system, not a postulate at all,
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although attempts to provide a proof all turned out to be circular. Yet the
controversial character of many primary premisses in no way deterred
investigators from claiming their soundness. The demand for arguments
that are unshakeable or immovable, unerring or infallible, inflexible in the
sense of not open to persuasion, indisputable, irrefutable or incontrovertible is expressed by different authors with an extraordinary variety of terms.
Among the most common are akinēton (immovable), used for example by
Plato at Timaeus 51e, ametapeiston or ametapiston (not subject to persuasion), in Aristotle’s Posterior Analytics 72b3 and Ptolemy’s Syntaxis 1 1
6.17–21, anamartēton (unerring), in Plato’s Republic 339c, ametaptōton
(unchanging) and ametaptaiston (infallible), the first in Plato’s Timaeus 29b
and Aristotle’s Topics 139b33, and the second in Galen, K 17(1) 863.3, and
especially the terms anamphisbētēton, incontestable (already in Diogenes of
Apollonia Fr. 1 and subsequently in prominent passages in Hero, Metrica
3 142.1, and in Ptolemy, Syntaxis 1 1 6.20 among many others) and anelegkton, irrefutable (Plato, Apology 22a, Timaeus 29b, all the way down to
Proclus in his Commentary on Euclid’s Elements 68.10).18
The pluralism of Greek mathematics thus itself has many facets. The
actual practices of those who in different disciplines laid claim to the title of
mathēmatikos varied appreciably. They range from the astrologer working
out planetary positions for a horoscope, to the arithmetical proofs and use
of symbolism discussed by Mueller and Netz in their chapters, to the proof
of the infinity of primes in Euclid or that of the area of a parabolic segment
in Archimedes. There was as much disagreement on the nature of the
claims that ‘mathematics’ could make as on their justification. One group
asserted the pre-eminence of mathematics on the grounds that it achieved
certainty, that its arguments were incontrovertible. Many philosophers and
quite a few mathematicians themselves joined together in seeing this as the
great pride of mathematics and the source of its prestige. But the disputable nature of the claims to indisputability kept breaking surface, either
in general or in relation to particular results. Moreover while there was
much deadly serious searching after certainty, there was also much playfulness, the ‘ludic’ quality that Netz has associated with other aspects of the
18
It is striking that the term anamphisbētēton may mean indisputable or undisputed, just as
in Thucydides (1 21) the term anexelegkton means beyond refutation (and so also beyond
verification). In neither case is there any doubt, in context, as to how the word is to be
understood. That is less clear in the case of the chief term for ‘indemonstrable’, anapodeikton,
which Galen has been seen as using of what has not been demonstrated (though capable
of demonstration) although in Aristotle it applies purely to what is incapable of being
demonstrated (see Hankinson 1991).
The pluralism of Greek ‘mathematics’
aesthetics that began to be cultivated in the Hellenistic period.19 In the case
of mathematics, there were occasions when its practitioners delighted in
complexity and puzzlement for their own sakes.
From a comparative perspective what are the important lessons to be
learnt from the material I have thus cursorily surveyed in this discussion?
The points made in my last paragraph provide the basis for an argument
that tends to turn a common assumption about Greek mathematics on its
head. While one image of mathematics that many ancients as well as quite
a few modern commentators promoted has it that mathematics is the realm
of the indisputable, it is precisely the disputes about both first-order practices and second-order analysis that mark out the ancient Greek experience
in this field. Divergent views were entertained not just about what ‘mathematics’ covered, but on what its proper aims and methods should be. The
very fluidity and indeterminacy of the boundaries between different intellectual disciplines may be thought to have contributed to the construction
of that image of mathematics as the realm of the incontrovertible – contested as that image was. But we may remark that that idea owed as much to
the ruminations of the philosophers – who used it to propose an ideal of a
‘philosophy’ that could equal and indeed surpass mathematics – as it did to
the actual practices of the mathematicians themselves.
It may once have been assumed that the development of the axiomatic–
deductive mode of demonstration was an essential feature of the development of mathematics itself. But as other studies in this volume amply show,
there are plenty of ancient traditions of mathematical inquiry that got on
perfectly well, grew and flourished, without any idea of the need to define
their axiomatic foundations. In Greece itself, as we have seen, it is far from
being the case that all those who considered themselves, or were considered
by others, to be mathematicians thought that axiomatics was obligatory.
This raises, then, two key questions with important implications for
comparativist studies. First how can we begin to account for the particular
heterogeneity of the Greek mathematical experience and for the way in
which the axiomatic–deductive model became dominant in some quarters?
Second what were the consequences of the hierarchization we find in some
writers on the development and practice of mathematics itself?
In relation to the first question, my argument is that there was a crucial
input from the side of philosophy, in that it was the philosopher Aristotle
who first explicitly defined rigorous demonstration in terms of valid
deductive argument from indemonstrable primary premisses – an ideal
19
Netz 2009.
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that he promoted in part to create a gap between demonstrative reasoning
and the merely plausible arguments of orators and others. Whether or how
far Aristotle was influenced by already existing mathematical practice is a
question we are in no position to answer definitively. But certainly his was
the first explicit definition of such a style of demonstration, and equally
clearly soon afterwards Euclid’s Elements exemplified that style in a more
comprehensive manner than any previously attempted.
From this it would appear that it was the particular combination of
cross-disciplinary and interdisciplinary rivalries in Greece that provided
an important stimulus to the developments we have been discussing.
Elsewhere in other mathematical traditions there was certainly competition between rival practitioners. It is for the comparativist to explore how
far the rivalries that undoubtedly existed in those traditions conformed to
or departed from the patterns we have found in Greece.
Then on the second question I posed of the consequences of the proposal
by certain Greeks themselves of a hierarchy in which axiomatic–deductive
demonstration provided the ideal, we must be even-handed. On the one
hand we can say that with the development of axiomatics there was a gain
in explicitness and clarity on the issue of what assumptions needed to be
made for conclusions that could claim certainty. On the other there was evidently also a loss, in that the demand for incontrovertibility could detract
attention from heuristics, from the business of expanding the subject and
obtaining new knowledge. This is particularly evident when Archimedes
remarks that conclusions obtained by the use of his Method had thereafter
to be proved rigorously using the standard procedures of the method of
exhaustion. If we can recognize – with one Greek point of view – that there
was good sense in the search for axioms insofar as that identified and made
explicit the foundations on which the deductive structure was based, we
should also be conscious – with another Greek opinion indeed – of a possible conflict between that demand for incontrovertibility and the need to
get on with the business of discovery.
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