Description
1. What role does Protagoras play in the dialogue?
What is the dialogue’s interpretation of “Man is the measure of all things”?
What other interpretations are possible?
A helpful resource is: https://plato.stanford.edu/entries/protagoras/#AllThinMeasManEpisInte
(Links to an external site.)
2. Explain the philosophical role played by either the wax tablet or the pigeons in Socrates’ arguments.
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Perception and Knowledge in Plato’s Theaetetus
Naly Thaler*
The Hebrew University of Jerusalem
Abstract
In this paper, I examine several key issues relating to the definition of knowledge as perception in the first
part of Plato’s Theaetetus. I begin by explaining the workings of the ‘secret doctrine’ of perception, which
is introduced in order to support the idea that perception is incorrigible (and hence worthy to the title
of knowledge), and then turn to examine the two refutations of the definition of knowledge as perception which appear at the end of the first part of the Theaetetus. I shall present and explain distinct lines of
interpretation pertaining to these passages and explore the consequences they attempt to draw regarding
Plato’s views on the perceptual world, the nature of perception and how these bear on his conception of
knowledge.
1. Introduction
The Theaetetus is devoted in full to a question that surfaces at various points in other Platonic
dialogues, namely, what is knowledge. But the attempt to mine the dialogue for Plato’s considered views on the matter is hampered not only by the fact that the dialogue ends in aporia, but
also by the fact that the bulk of the discussion is devoted to examining and refuting suggestions
which are liable to appear as obvious non-starters: of the dialogue’s 68 Stephanus pages, 36 are
devoted to the suggestion that knowledge is perception, and a further 14 to the suggestion that
knowledge is (mere) true judgement, two ideas which any reader of Plato would likely find
unpromising. Only after discussing these suggestions at length and determining their inadequacy
does the dialogue proceed to devote a relatively scant ten pages to the more promising claim
that knowledge is true judgement with the addition of an account. But even this final and
potentially fruitful suggestion is then couched in terms of a highly specific ontological theory
Plato seems to have little sympathy for and is ultimately refuted.
One can discern two basic approaches which interpreters of the dialogue have tended to
take in light of this feature of the dialogue. The first is not to be discouraged by the negative
conclusions at various stages in the dialogue and treat it as a treasure-house for sophisticated
(and considered) Platonic views in the field of epistemology. Moreover, since some of the
arguments in the dialogue contain theses that are alien to what is often associated with Plato’s
‘middle period’ views (especially in dialogues such as the Phaedo and Republic), this line of
interpretation takes the Theaetetus to contain important innovations and revisions in Plato’s
views about such matters as the nature of perception and the scope of knowledge. Book
length treatments of the dialogue that accord with this approach are McDowell, Bostock
and Burnyeat.
The second approach claims that we should not treat the dialogue’s arguments as vehicles for
conveying Plato’s considered views at all. According to this interpretation, Plato’s considered
views are those found in the middle period dialogues; and while they are conspicuously absent
from the Theaetetus, Plato nevertheless wrote the dialogue while taking into account his readers’
familiarity with them. In that way, the various failures of the ‘un-Platonic’ suggestions in the
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dialogue regarding the nature of knowledge serve as implicit proofs of the truth of Plato’s own
familiar views (such as the existence of non-material forms) on the subject. Book-length
treatments of the dialogue written under this approach are those of Cornford and Chappell.1
The present paper will concentrate on the dialogue’s first definition of knowledge, which
identifies it with sense perception. I begin in Section 2 with a discussion of Socrates’ development of the thesis that knowledge is perception, which proceeds first by equating it with
Protagoras’ man-measure dictum, and then introducing a ‘secret doctrine’ which Protagoras
supposedly held in order to justify his man-measure dictum, according to which the world is
in constant f lux. From there I proceed to discuss Socrates’ two refutations of the thesis that
knowledge is perception. Section 3 deals with the first refutation which attacks the thesis
when considered under the specific ontological commitments of the f lux theory.
Section 4 deals with the subsequent refutation of the thesis which attacks it under more
ordinary ontological commitments, ones that do not presuppose a world in f lux. In laying
out the various interpretations of these two refutations I shall show how some commentators
see them as revising Plato’s earlier views about the relation between knowledge and the
perceptible world, whereas others find in them a reaffirmation of the middle-period’s
apparently disparaging view about perception and its insistence on the exclusive relation
of knowledge to the intelligible world.2
2. The Definition of Knowledge as Perception and its Development
The suggestion that knowledge is identical with perception should immediately appear odd
to readers of Plato, since in dialogues such as the Phaedo and Republic he tends to contrast the
cognitive capacities associated with the faculty of perception with those that belong to the
faculty responsible for knowledge, namely, intellect. And one cannot evade this difficulty
by supposing that the suggestion is representative or becoming of the views of one of the
dialogue’s participants: Theaetetus himself is a budding mathematician at the dramatic date
of the conversation, a fact which makes him an unlikely candidate for entertaining the
suggestion that knowledge is simply perception. So one question which should immediately
strike any reader of the dialogue is what philosophical benefits Plato thinks are to be gained
from a serious examination of this clearly false suggestion.
The explicit rationale behind the initial presentation of Theaetetus’ definition is the rather
opaque claim that ‘he who knows something perceives what he knows’ (151e). Socrates immediately takes over and develops the definition in two unexpected directions. He first identifies it
with Protagoras’ claim that man is the measure of all things, meaning by this roughly that all
judgments are true for those who make them. Socrates then claims that Protagoras himself held
a secret doctrine about the nature of reality, according to which everything is in constant
change. This doctrine, it soon turns out, is supported by an equally esoteric perceptual theory
according to which perceptible properties do not inhere in objects (as we would be inclined
to think) but are rather transitory phenomena that are unique to, and fully determined by,
the interaction of a specific perceiver with a specific object.
The issue of the precise relation between the three theses (Theaetetus’ definition, Protagoras’
man-measure doctrine and the secret doctrine) is subject to scholarly controversy. Burnyeat,
1982; 1990 (9–10), has been inf luential in claiming that the man-measure doctrine constitutes
a sufficient and necessary condition for the truth of Theaetetus’ thesis and that the theory of f lux
constitutes a sufficient and necessary condition for the truth of the man-measure doctrine, in
effect making the three theses mutually entailing. This interpretation has been criticized by
Lee (88–92), who prefers to construe the relation more loosely than strict mutual entailment.
According to Lee, the Secret doctrine is simply one possible way to support and bolster
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Theaetetus’ definition and is not entailed by it in any way (a view which sits well with the fact
that Socrates takes care to refute Theaetetus’ definition after the secret doctrine has
been refuted).
For the present purpose, it will be helpful to concentrate on the fact that equating Theaetetus’
definition with Protagoras’ man-measure doctrine immediately places the emphasis of the
discussion on one particular concept which seems inseparable from, but not definitive of
knowledge, namely, truth. This connection comes to the fore in Socrates’ claim that if man is
in fact the measure of truth in regard to his perceptions (Protagoras’ thesis applied to the sphere
of perception), that entails that they are always true for him. And since it is a defining feature of
knowledge that it is always of what is true, it appears that perception does in fact have a claim to
the status of knowledge (152b–c). From that perspective, the secret doctrine of perception
which is announced immediately following the introduction of Protagoras’ view can (as
Lee suggests) be thought of as a theory which attempts to give one possible account of how it
is that perceptions always turn out to be true.
The theory of perception (described in 153d–154a and 156a–157c) is intended to provide
detailed backing for the claim that the world is in constant change and an explanation of why
that entails that all our perceptions are true. It achieves this double feat by claiming that any
act of perception is merely one part of a complex event in which a corresponding perceptual
property is generated in the object perceived. Perceptual properties, it turns out, are not
independent features of the physical world, features which perceivers gain access to by
means of their sensory apparatus. If that had been the case, there would have been a margin
for error in perceivers’ grasp of these properties, and perception would have been corrigible.
Rather, the theory construes perceptual properties as the product or (in the theory’s
own terminology) ‘offspring’ of a specific and momentary interaction between entities of
two types: perceiving subjects and perceived objects. Perceptual properties come into being
at the same time as, and in exact correspondence to, the perceptions we have of them. Thus,
the theory refers to perceptions and perceptual properties as ‘twin offspring’, whose ‘parents’
are objects and perceivers that happen to come into each other’s vicinity. What determines
the precise quality of each pair of offspring (a particular shade of ‘whiteness’, and a corresponding ‘sight-of-whiteness’) in this generative event is the state in which the object and
its perceiver happen to be: for example, to Socrates when sick the wine will appear, and
hence be, bitter; to Socrates when healthy it will appear and be sweet.
Now, since according to the theory the parents (qualified objects and their perceivers) of the
twin offspring are in constant change, it turns out that any instance of these twins, i.e. every
perceptual property and every corresponding perception, will have a distinct quality. The
unique quality of these perceptions and perceptual properties in turn accounts for the fact that
no two objects have the same appearance, and no two subjects have the same perception. From
this, it follows that my perceptions are always of what is ( for me) and that they are necessarily
incorrigible: they are of what ‘is’ since the fact that I have a particular perception necessarily
entails that the precisely corresponding property does in fact exist in the object. And they are
incorrigible, since any supposedly conf licting perception anyone else might have of that object,
is of a different property altogether, and hence not in fact in conf lict with my perception.
From the description given above, it is natural to assume that the secret doctrine is committed
to a relatively clear and intelligible causal chain: objects and perceivers undergo constant physical
changes. When these constantly changing objects and perceivers meet, they produce perceptions and perceptible properties whose quality is determined by the precise (and unique) physical state they were in at the time of their encounter. And these properties and perceptions then
qualify the objects that engendered them, so that e.g. sick Socrates who tastes the wine comes to
be qualified by having a bitter sensation, and the wine itself comes to be qualified as bitter.
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But things become somewhat trickier when we notice that at various points when drawing out the implications of the theory (such as in 159a–c; 160a–d), Socrates’ claims seem to
imply that the objects and perceivers whose contact with each other produces perceptual
properties and perceptions have no independent nature or properties of their own. All their
properties turn out to be relational ones which are determined by their interactions with
perceivers (in the case of objects) and perceptible objects (in the case of perceivers). If that
is true, then the f lux which aff licts objects and perceivers is simply the result of their mutual
interactions (and not some independent fact about them), and perceivers and perceived objects
turn out to be nothing more than momentary collections of perceptions and perceptible properties. Thus, the sickness of Socrates, which we previously construed as a cause of his perception
of the wine as bitter, is itself determined by, and reducible to, that perception (and others like it);
the difference between ‘sick Socrates’ and ‘healthy Socrates’ will correspondingly be reduced to
a difference between their respective perceptions; and, in this difference between the healthy
Socrates and the sick one will be sufficient evidence that they must count as two distinct individuals. Among commentators who claim that the theory does away with objects and perceivers
and leaves us with mere collections of properties and perceptions are Burnyeat, 1982; 1990
(18–19), Day (65–70), Bostock (68–78), Sedley (44–48). Commentators who support the
priority of objects are Matthen (1985), who argues that the theory becomes incoherent if devoid
of objects and perceivers which are distinct from their perceptual properties and perceptions,
and Brown (206–209), who claims that there is no real textual support for the ‘bundle of
properties’ view.
In line with its ontology of change, the theory also introduces several revisionary linguistic
prescriptions designed to bring everyday talk about reality into conformity with its true
nature. First and foremost of these revisions concerns the eradication of the verb ‘to be’:
the theory claims that we must replace it with the verb ‘comes into being’ so as not to convey
the false impression of stability in nature. The theory also prohibits the use of demonstratives
such as ‘this’ and ‘that’ (apparently, since they imply spatial and temporal stability), and of possessives such as ‘mine’ (which imply the stable identity of the perceiver). Finally, it asks us to
refrain from the use of terms which signify many things collected together, such as ‘man’ or
‘dog’ (probably since they imply that the collection of properties has an underlying
enduring nature).3 It should become apparent that these restrictions severely limit the possibility
of conf lict between any two statements about perception: without the ability to specify a determinate subject (no demonstratives or kind terms) or make any commitment that a perceived
property qualifies it (no ‘is’), statements about perception will not cross the threshold of
determinacy required for them to conf lict with each other.
3. The Refutation of the Definition of Knowledge as Perception Under the Hypothesis of Flux (181b–183c)
The argument against the thesis that perception is knowledge, construed under the unique
assumptions of the f lux theory, proceeds by means of a reductio: Socrates is able to show that
if the f lux-hypothesis entails that things are both in constant qualitative alteration and in
constant motion (the two possible types of f lux),4 accepting it will have the consequence that
any claim one makes will entail its contradictory as well. The refutation begins with the claim,
which is presented as a direct entailment of the inclusion of both motion and alteration in the
theory’s notion of f lux, that it is impossible to ascribe a determinate name to a particular
property such as whiteness; it proceeds to show that this makes it impossible to give any
determinate sense to general qualitative concepts such as ‘colour’ or ‘sound’; this is said to lead
to the collapse of specific perceptual concepts such as ‘sight’ or ‘hearing’, which is then said to
entail that the more general concept ‘perception’ is itself indeterminate; from there the
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conclusion is clinched: Socrates claims that any statement about this indeterminate subject, such
as ‘perception is knowledge’, entails its contradictory as well.
While there is general scholarly agreement that the argument is meant to expose the need
to posit some degree of stability in the world in order to account for the phenomenon of
language, scholars are at odds as to where precisely Plato intends us to locate this stability.
Some commentators, such as Silverman, Chappell 137–140, follow Cornford’s lead
(95–101) and claim that Plato himself accepts the theory’s claim that bodily reality and
sensory perceptions are in constant change. Accordingly, they claim that showing that under
conditions of ubiquitous f lux, language itself will collapse is meant to expose the need to
posit extra-physical entities (i.e. Platonic forms) which are unchanging and which will serve
as a stable reference for linguistic concepts. The justification for this reading offered by
Silverman and Chappell (expending on Cornford’s view) is that the only way to make sense
of the argument which takes f lux to entail the total collapse of language is to construe this
f lux as pertaining not merely to particulars but to abstract general properties as well. That is
because f lux which pertains merely to perceptual particulars will at most lead to the collapse
of statements about the perceptual world but will leave untouched general statements about
abstract truths, such as ‘perception is knowledge’. According to this interpretation then, the
f lux theory and its refutation are a means of exhibiting the consequences that follow on
the assumption that general terms have no stable objects of reference. Since these consequences are at odds with the facts (the existence of language with stable word and
sentence meaning), the reader is left to conclude that immutable objects of reference for
general terms, i.e. Platonic forms, do exist.
Other commentators, such as Owen (72–73), Bostock (99–110), Burnyeat, 1990 (46–52),
Sedley (99–102) claim that the argument’s conclusion is not meant to bring out the necessity
of positing a stable, non-physical realm.5 According to their reading, the theory’s f lux pertains
only to physical reality, and its refutation is intended to show that the realm of perceptual reality
must itself include a sufficient degree of stability to allow determinate meanings for linguistic
utterances. As mentioned above, interpretations of this type have been taken to be vulnerable
to the objection that f lux which pertains only to particular objects cannot lead to the failure
of utterances about general concepts (most notably, ‘perception is knowledge’). One way to
defend this interpretation against such a challenge6 is to emphasize the f lux theory’s earlier
ban on familiar locutions that pertain to perceptual experience. According to this reading,
one of the lessons of the refutation of the f lux-theory is that the intelligibility of more general
concepts (such as ‘colour’, ‘sight’ and ‘perception’) depends on the prior intelligibility of
determinate utterances about perceptual reality (such as ‘this stone is white’) of the sort which
the theory explicitly denies under its conception of physical f lux. According to this reading,
the precise degree of stability Plato thinks must be attributed to perceptual reality is defined
by the sort of locutions that must be applicable to it. These minimally include both demonstratives and the verb ‘to be’.
4. The Second Refutation of the Definition of Knowledge as Perception (183c–186e)
After showing that the thesis of f lux makes Theaetetus’ suggestion that perception is knowledge untenable, Socrates turns to refute the suggestion under everyday, non-philosophical,
assumptions that seem diametrically opposed to those of the f lux theory: as the refutation
makes clear, the definition is now construed under the assumption that perceptual properties
are inherent in physical objects, that nature is characterized by the sort of stability attributed
to it by folk ontology and that perceptual judgments encapsulated in locutions involving the
verb ‘to be’, are applicable to it.
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The refutation has aroused a great deal of interest over the past 50 years and has been the
subject of considerable debate. The argument begins by claiming that we perceive a distinct
feature of reality through each of the particular senses (sight perceives colours, hearing sounds,
etc.) and that no sense can perceive the objects of another. The argument then points out that
since we grasp certain features that are common to objects of several distinct senses, such as that
both blue and F# are that they are different from each other and are both identical to themselves, it
is necessary to infer some central apparatus which is conscious of the various perceptions and
which has the capacity to grasp the features that are common to them. The central apparatus
in question is agreed to be the soul, and the grasp of the properties in question the result
of its own independent activity (rather than an activity conducted through one of the
particular senses). The argument then proceeds to claim that since one of these common
features, namely being, must be grasped by anyone who is to attain the truth about anything,
and since if one is ignorant of the truth about something one cannot be said to know it, sense
perception itself cannot be knowledge.
The scholarly debate around this argument centres on the precise relation between the
soul’s grasp of the property ‘being’ and the attainment of truth which is said to be necessary
for knowledge. According to one line of interpretation, originating with Cooper and
supported by Modrak, while sense perception allows us to grasp the appearance of objects,
such as that the rose seems red or the stone hard, it cannot be used to determine whether this
appearance has any objective truth in it, i.e. whether the rose really is red or the stone really is
hard. All the senses can do on their own is provide us with subjective judgments about the
appearance of things. Since knowledge must contain some element of truth about the
outside, inter-subjective world and since truth requires the sort of calculation and positive
investigation which the senses on their own cannot provide, it turns out that sense perception on its own cannot attain knowledge.
The second interpretation limits even more the cognitive capacities of sense perception.
According to this interpretation ( for which see Burnyeat 1976, Frede, Lorenz (74-94)), perception on its own cannot convey even the minimal information required for the (subjective)
judgement that the rose has a red appearance or that stone is hard to the touch. That is because
even these basic perceptions must be couched in some form of judgement, and such judgments
require a propositional structure which sense perception on its own simply cannot provide.
Since any meaningful perceptual judgement must take the form of a proposition such as ‘this
rose is red’ and since being (which is signified by the verb ‘to be’ in the proposition) was agreed
to be grasped by the soul working through itself, it turns out that any meaningful act of perception, i.e. of the kind which crosses the threshold of having a truth value, requires something
over and above the mere use of the bodily senses.
The latter reading seems to have substantial implications for Plato’s views about the nature
of perception. In the middle-period dialogues, Plato seems committed to the view that
the faculty of perception has an integral capacity, independent from reason, to produce
judgments about the physical world. This commitment is found, for example in Rep. 524
where the senses are explicitly described as providing the perceiver with conf licting
statements about the object before her. And while the Rep. passage is ultimately meant to
bring out the fact that the senses require the help of reason in order to solve this conf lict,
it nevertheless describes them as independently conveying the sort of information which
the Theaetetus passage, under the second interpretation, denies they are able to provide. So
while the first interpretation preserves the ability of the perceptual faculty to produce at least
some low-level judgments (such as those that attribute perceptible properties to objects)
independently of reason, the second interpretation turns any perceptual judgement into a
hybrid activity that involves both perception and reason.7
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But aside from this difference, it is important to note a point of agreement between the two
interpretations: according to both, the properties grasped by the soul through itself are not
independently existing intelligible entities, such as Platonic forms might be thought to be.
Rather, these non-perceptual properties are aspects of perceptual reality: what the soul grasps
is not being itself but the being-of-the-hardness of the stone; and it does not grasp difference as such
but the difference-of-blue-from-f#. This feature of the argument, about which there is close to a
general agreement today,8 constitutes a substantial obstacle to Unitarian interpretations such
as Cornford’s that attempt to find an underlying metaphysics of the kind familiar from the
Phaedo and Republic beneath the surface of the Theaetetus.
9 At least one reason for this is that
if the property being, which is said to constitute the basis for our grasp of truth and hence
of knowledge, belongs to perceptual objects, this might reasonably be taken to imply that
knowledge is a form of cognition which pertains to bodily reality. Instead of the middle period
view according to which being and hence knowledge belong solely to objects in the intelligible
realm, we now seem to find the view that the objects of perceptual reality fulfil the necessary
conditions for being objects of knowledge.
Such readings of the first part of the Theaetetus, according to which the dialogue contains a
commitment to stability in the bodily world and to the applicability of both being and
knowledge to it, naturally raise questions about how precisely Plato construed the relation
between perception and knowledge when writing the dialogue (after all, despite the
elevation in the status of perceptual reality, the Theaetetus makes patently clear that Plato
did not simply identify perception with knowledge). In order to describe possible avenues
of interpretation along these lines, it will first be necessary to give some account of the
subsequent two sections of the dialogue. The first of these attempts to identify knowledge
with (mere) true judgement; the one that follows it suggests that true judgement with the
addition of an account will be equivalent to knowledge. In the sequel to this paper, I
shall deal with these two suggestions in turn and then attempt to spell out some general
conclusions regarding the relation between the three suggestions.
Short Biography
Naly Thaler received his PhD from Princeton University and is currently an assistant professor at
the Hebrew University of Jerusalem. His research interest is Ancient Philosophy, especially
Plato and Aristotle. He has published in the Journal of the History of Philosophy, Oxford
Studies in Ancient Philosophy, and Phronesis.
Notes
* Correspondence: Philosophy, The Hebrew University of Jerusalem, Jerusalem, Israel. Email: thaler.naly@mail.huji.ac.il
1 A third way, suggested by Sedley, attempts to mitigate the apparent discrepancy between the assumptions at play in the
arguments of the Theaetetus and the metaphysics familiar from the middle period dialogues by distinguishing two levels at
which these arguments are meant to be read: at the first, ‘surface’, level they are meant to represent the philosophical
sophistication achieved by the historical Socrates and thus make use only of such means as would have been available to
him in a dialectical confrontation with various subjectivist or relativist views about truth and knowledge. This explains
why the Theaetetus makes no mention or appeal to separable Platonic forms even at those junctions where this might
have seemed most appropriate. At the same time however, the arguments are meant to resonate with readers of Plato’s
middle period dialogues and expose how Socrates’ own metaphysically innocuous ideas were a natural precursor to
familiar Platonic metaphysics.
2 For the purpose of this paper I leave to the side the dialogue’s discussion of Protagoras’ view that man is the measure of all
things (i.e. of the truth of all his judgments, not merely those relating to perception) and focus on issues pertaining specifically
to perception and its relation to truth.
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3 Commentators who do not subscribe to the ‘bundle of properties’ view claim that the claim about things collected
together refers to individual objects and general species. According to this reading, the prohibition on saying ‘dog’ would
be aimed at making claims that have the general canine type as a subject and not at ones that identify a particular as falling
under that species.
4 The theory itself had originally made no mention of two types of flux, and there is scholarly disagreement as to whether
saddling it with a commitment to both types of change – a feature which seems necessary for its refutation – is warranted by
the theory’s own commitments or whether it is an unfair move made by Socrates merely to facilitate the theory’s refutation.
Commentators who think that the radicalization is required by the theory itself for the purpose of preserving the infallibility
of perception are Burnyeat, 1990 (49-51), Denyer (102-103), Sedley (92-93); those who claim that the theory agrees to
radicalize flux simply because of its claim that flux is complete are Brown (212), Lee (116-117), Thaler, 2013 (24-26). 5 Though according to Sedley (99-102), while the argument itself does not appeal to extra-physical reality, it is supposed to
put readers in mind of the option (taken by Plato himself) of doing so.
6 For which see Thaler.
7 For the claim that this argument is the first instance of the use of ‘perception’ in such a limited sense, see Frede. 8 It was first noted by Cooper. 9 And so it is in fact rare to find Unitarian interpretations of the dialogue that offer a reading of this argument that is
consistent with that general thesis. For an attempt at such an interpretation see Chappell (147). Sedley (105-117) agrees
that the argument treats the ‘common properties’ as inherent in perceptual objects but claims that readers are meant to be
put in mind of their Platonic, separable, counterparts.
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