Aristotle’s Criticism of Plato and the Limits of the Argument from Diversity
Abstract
In Nicomachean Ethics I.6, Aristotle criticizes the Platonic Form of the Good by appealing to the priority and posteriority of goods, their distribution across different categories, the plurality of sciences investigating them, differences among goods pursued for their own sake, and the apparent practical uselessness of universal knowledge. This paper argues that these considerations do not suffice to exclude a common explanatory principle of goodness. Its central distinction is between differences among things that are good and differences in the respect in which they are good. Although the former do not establish the latter, neither does common predication independently establish a univocal essence. The decisive question is therefore what kind of unity Aristotle’s arguments exclude. Aristotle himself acknowledges the possibility of unity through a common source, a common end, or analogy. These forms of intelligible unity need not be generic, and their possibility raises a question about whether the Platonic Good might supply their explanatory ground. The paper further argues that the plurality of practical sciences does not preclude general knowledge of goodness, and that the insufficiency of universal knowledge for particular action does not establish its uselessness. It distinguishes the burden of excluding a proposed explanatory unity from the burden of demonstrating that a separately existing Form provides it. The conclusion is limited but significant: Aristotle’s objections impose substantial constraints upon a Platonic theory of goodness, but they do not, without further premises, establish the impossibility of a common, non-generic explanatory principle.
1. Introduction
In Nicomachean Ethics I.6, Aristotle undertakes a criticism of the Platonic Form of the Good. Although he acknowledges the difficulty of criticizing a theory advanced by those close to him, he insists that philosophical inquiry must place truth above personal allegiance (1096a11–17).
His objections are substantial. Goods stand in relations of priority and posteriority; goodness is predicated across different categories; different sciences investigate different goods; even goods pursued for their own sake appear to possess different accounts of goodness; and knowledge of a universal Good seems to contribute little to the practical inquiries concerned with particular ends.
These arguments challenge both the metaphysical unity of goodness and the practical relevance of knowing it.
Yet a fundamental distinction remains insufficiently established in the argument from diversity: the distinction between differences among things that are good and differences in the respect in which they are good.
Aristotle does not merely observe that substances, qualities, quantities, and relations differ. He predicates goodness within each of these categories. Virtue is good as a quality, the appropriate amount as a quantity, and the useful as a relation.
The question is what follows from these predications.
Does the fact that different things are good in different ways establish that their goodness cannot depend upon a common intelligible principle?
We shall argue that it does not, at least without further premises.
This criticism must be distinguished from a positive defense of the Platonic theory. We do not attempt to prove that the Form of the Good exists, that goodness is one genus, or that all uses of good are univocal. We ask whether Aristotle’s objections establish that a common explanatory principle of goodness is impossible.
The argument thus distinguishes two burdens.
The first is the burden of exclusion: Aristotle must show why the diversity of goods precludes the kind of unity attributed to the Platonic Good.
The second is the burden of explanation: a defender of Plato must show that a separately existing Form actually provides a substantive explanation of the differentiated goodness of particular things.
The present paper addresses primarily the first burden. The failure of an argument against a theory does not establish the truth of that theory. But neither does the absence of a positive demonstration justify an inference to its impossibility.
Our thesis is that Aristotle’s objections establish important constraints upon any acceptable theory of goodness without conclusively excluding the relevant Platonic explanatory possibility.
2. Common Determination and the Argument from Diversity
Consider the following principle:
No two things that are F differ insofar as they are F.
Two squares may differ in size, orientation, material, or location without differing insofar as they are squares. Two pieces of gold may differ in weight, shape, and temperature without differing insofar as they are gold.
These examples distinguish differences among instances from differences under a common determination.
Call this the Principle of Common Determination.
Its significance is primarily dialectical.
The principle does not establish that every predicate signifies one univocal essence. Nor does it establish that everything of which a predicate is truly affirmed possesses one identical intrinsic property.
Consider healthy. A person may be healthy by possessing health, a diet by preserving health, and a complexion by indicating health. The predicate applies through different relations to a common explanatory focus.
Accordingly, the mere fact that two things are called healthy does not establish that they possess health in the same way.
The same caution applies to goodness.
The principle therefore does not settle whether goodness is univocal, analogical, focal, or accidentally homonymous.
Rather, it identifies an inferential constraint:
Differences among the subjects of a predicate do not, merely as such, establish the absence of a common intelligible account of its application.
This constraint is especially important because Aristotle himself supplies the relevant predications. His argument begins from things he identifies as good across different categories.
The issue is not whether those things are good, but what their differences establish about goodness.
To exclude a common explanatory principle, Aristotle must show either that the relevant uses of goodness possess no unity of the proposed kind or that the Platonic theory necessarily requires a form of unity already shown to be impossible.
Neither conclusion follows merely from categorical diversity.
3. The Argument from the Categories
Aristotle observes that goodness is predicated in the categories of substance, quality, quantity, relation, time, and place (NE I.6, 1096a23–29). God or intellect is good in substance, virtue in quality, the appropriate amount in quantity, and the useful in relation.
His argument draws upon a central feature of Aristotelian metaphysics: being is not one genus encompassing the categories.
Since goodness is predicated across the categories, Aristotle reasons that goodness cannot constitute one universal genus common to them all.
This is a serious objection to a conception of the Good as a highest genus under which every particular good falls.
But it does not establish the impossibility of every common explanatory principle.
We must distinguish two forms of unity.
Generic unity obtains when different things belong to one genus through a common generic account.
Explanatory unity obtains when different things are intelligibly related through a common source, end, structure, or explanatory principle, without necessarily belonging to one genus.
The denial of generic unity does not entail the denial of explanatory unity.
Aristotle’s own metaphysics supplies a relevant comparison. In Metaphysics IV.2, being is said in many ways, but its different senses remain systematically related through reference to one primary meaning (1003a33–b19).
This does not establish that goodness possesses the same structure. It does, however, demonstrate that Aristotle recognizes intelligible unity without generic unity.
The categorical argument therefore excludes a particular conception of universality without necessarily excluding a common explanatory ground.
An Aristotelian may reply that Plato’s Form requires precisely the univocity Aristotle rejects.
This is the strongest version of the objection.
But it introduces an additional interpretive premise: that the Platonic Good must be a genus or possess one essence-specifying definition immediately applicable to all particular goods.
That premise requires defense.
Plato’s Good may instead be understood as an explanatory principle accounting for why different things possess their respective modes of goodness, without requiring that those modes be identical.
Whether this interpretation is ultimately correct remains disputed. But the categorical argument does not exclude it merely by rejecting generic unity.
The outstanding question is whether Aristotle’s argument reaches the conception of explanatory unity actually required by the Platonic theory.
4. Priority and Posteriority
Aristotle’s first objection concerns goods standing in relations of priority and posteriority (NE I.6, 1096a17–23).
He appeals to the Platonists’ reluctance to posit a common Form over things ordered as prior and posterior. Since goodness is predicated of such things, he concludes that there cannot be one Form corresponding to them all.
This argument possesses a force not reducible to categorical diversity. It appeals to a restriction attributed to the Platonists themselves.
If the Platonists accepted an unrestricted prohibition on Forms over ordered series, Aristotle’s objection would expose a tension within their theory.
Nevertheless, two questions remain.
First, what precisely is the restriction, and why must it apply to the Good?
Second, does the priority of some goods over others exclude a common explanatory principle, or only a Form conceived as a genus common to coordinate instances?
Things may stand in relations of asymmetrical dependence while remaining intelligibly related through a common principle.
Indeed, an explanatory principle may account for the distinction between primary and derivative instances precisely by explaining their dependence.
The Platonic Good is plausibly understood as explanatorily prior to particular goods, rather than as one member of their class.
Such priority need not conflict with differentiated modes of goodness.
The objection therefore establishes a difficulty for a particular interpretation of the theory of Forms, but does not independently demonstrate that every explanatory unity encompassing prior and posterior goods is impossible.
5. The Plurality of Sciences
Aristotle further observes that different sciences investigate different goods (NE I.6, 1096a29–34).
Medicine investigates health, generalship victory, and other disciplines their respective ends. Even the appropriate moment or appropriate quantity is investigated differently in different fields.
The argument may be reconstructed as follows:
1. If all goods fell under one Form, they would belong to one science.
2. Goods belong to different sciences.
3. Therefore, all goods do not fall under one Form.
The decisive question concerns the first premise.
Why should the existence of a common explanatory principle require that every particular investigation of its instances belong to one science?
Aristotle himself recognizes general inquiries that do not replace the specialized disciplines.
The Posterior Analytics, for example, investigates the nature of scientific knowledge and demonstration without supplying every theorem of geometry or every principle of medicine.
A general account of scientific knowledge is compatible with a plurality of particular sciences.
Likewise, a general account of goodness might illuminate what different practical inquiries share without replacing their particular principles.
The analogy has a limitation. A general theory of demonstration is methodological, whereas the Platonic Good is supposed to be a substantive explanatory principle.
The existence of the former does not establish the existence of the latter.
But our argument requires no such demonstration.
It requires only that plurality among particular sciences does not itself establish the impossibility of a more general inquiry into their objects insofar as those objects are good.
Aristotle must explain why one common explanatory principle would require the elimination of distinct sciences of its particular instances.
Without that additional premise, the argument from scientific plurality is inconclusive.
6. The Diversity of Intrinsic Goods
Aristotle anticipates a possible Platonic reply: perhaps the Form of the Good concerns only things good in themselves, while useful or productive goods are called good derivatively (NE I.6, 1096b8–14).
He therefore restricts the inquiry to intrinsic goods.
But even here, he argues, goodness does not possess one common account.
Honor, wisdom, and pleasure differ in their goodness, rather than merely in accidental characteristics (NE I.6, 1096b14–25).
This is a stronger argument than the categorical objection.
It does not merely identify differences among goods. It claims that their accounts differ precisely insofar as they are good.
Accordingly, the Principle of Common Determination cannot by itself answer the objection.
If Aristotle has established that honor, wisdom, and pleasure possess irreducibly different accounts of goodness, the inference from diversity to non-univocity is no longer based merely on differences among their subjects.
Nevertheless, a distinction remains.
Different intrinsic goods may possess different proximate explanations of their goodness while depending upon a more general explanatory principle.
Wisdom may be good as an excellence of intellect, pleasure as the fulfillment or accompaniment of certain activities, and honor as recognition of excellence.
Their immediate explanations differ.
But it does not follow that no further principle could explain why these different modes of excellence, fulfillment, and recognition are good.
To illustrate the possibility, consider a candidate account according to which goodness consists, at a sufficiently general level, in the excellent realization of a thing’s characteristic capacities or ends.
On such an account, wisdom might be good through the excellent exercise of intellect, pleasure through its relation to fulfilled activity, and honor through the appropriate recognition of excellence.
The examples would not possess one identical proximate definition of goodness. Yet their goodness might be intelligible through a common explanatory structure.
This proposal is illustrative rather than established. It may fail to account for every kind of good, and it does not independently vindicate a separately existing Form.
Its significance is logical: different proximate accounts do not automatically exclude a common higher-order explanation.
The distinction is therefore between:
1. A single identical account immediately defining every intrinsic good.
2. A common explanatory principle accounting for differentiated modes of goodness.
Aristotle’s objection may defeat the first without defeating the second.
An Aristotelian may reasonably demand that the proposed higher-order principle be specified and shown to explain something substantive.
That demand is legitimate.
But it concerns the adequacy of the proposed explanation, not its logical impossibility.
The Platonic defender must eventually show that the Good provides more than an empty common name. Aristotle, conversely, cannot establish that no such explanation is possible merely by demonstrating diversity among proximate accounts.
The objection therefore narrows the kind of unity available to the Platonist without conclusively excluding every explanatory unity.
7. Aristotle’s Recognition of Non-Generic Unity
The preceding distinction receives important support from Aristotle’s own discussion.
At Nicomachean Ethics I.6, 1096b26–29, Aristotle asks whether goods are so called because they derive from one good, contribute toward one good, or are related by analogy.
These possibilities are significant because none necessarily requires generic unity.
Things may derive from one source without belonging to one genus. They may contribute toward one end without sharing one intrinsic essence. They may also exhibit analogical relationships across different categories.
Aristotle’s own example compares the relation of sight to the body with that of intellect to the soul.
The things compared differ, but their relationship possesses an intelligible correspondence.
Analogy thus provides a form of structural unity without requiring identity of realization.
The same holds for unity through a common source or end.
Aristotle does not definitively endorse one of these alternatives in I.6. He sets the question aside as more appropriate to another inquiry.
Nevertheless, the passage is significant for the dialectical framing of his criticism.
Aristotle himself recognizes that the plurality of goods need not amount to accidental homonymy.
The relevant question is therefore not whether goodness possesses any intelligible unity, but what kind of unity it possesses and how that unity is explained.
This raises a further question for Aristotle’s criticism of Plato:
Why could the Platonic Good not serve as the explanatory ground of a differentiated, non-generic unity among goods?
Aristotle might answer that such a Form explains nothing beyond the particular structures already available to inquiry.
He might argue that a separate principle is metaphysically unnecessary or that analogy and focal reference provide sufficient explanations without it.
These are serious possible objections.
But they are distinct from the inference that categorical diversity excludes the relevant unity.
Aristotle’s acknowledgment of non-generic unity therefore limits the conclusion that can be drawn from his earlier arguments.
The existence of different modes of goodness is compatible, at least in principle, with their dependence upon one explanatory ground.
8. Universal Knowledge and Practical Usefulness
Aristotle concludes his discussion by questioning the practical relevance of the universal Good (NE I.6, 1096b30–1097a14).
A physician must understand how to heal particular patients. A general must understand how to secure victory in particular circumstances.
Knowledge of the Good itself does not appear to supply this specialized knowledge.
But this argument risks confusing the insufficiency of universal knowledge with its uselessness.
Universal knowledge need not determine every particular judgment in order to contribute to inquiry.
Consider the definition of a square.
Knowing what a square is does not determine the dimensions or location of every particular square.
Nevertheless, such knowledge enables us to identify squares, derive their properties, and distinguish their essential characteristics from accidental variations.
Its generality is not a defect. It is what permits its application across particular cases.
The same possibility arises for goodness.
A universal account might contribute to practical inquiry in three distinct ways.
Explanation. It might illuminate why different things are good, identifying relationships among their differentiated forms of goodness.
Classification. It might help distinguish genuine goods from merely apparent goods, or intrinsic goods from those valuable only as means.
Evaluation. It might assist in comparing, qualifying, and ordering the ends pursued by particular sciences.
The third function is especially important.
Medicine investigates health and its preservation. But the successful pursuit of health does not exhaust every question concerning the good of the patient.
A medical intervention may be technically successful while conflicting with other goods relevant to a person’s life.
Questions about justice, suffering, autonomy, and broader flourishing may require evaluation beyond the specialized competence of medicine.
A general account of goodness could help situate health within an ordering of human goods.
This possibility is particularly significant because Aristotle himself regards politics as architectonic: it orders subordinate disciplines toward a comprehensive human end (NE I.2, 1094a26–b11).
Aristotle may answer that political science and practical wisdom provide this ordering without requiring a separate Form.
That answer is available to him.
But it concedes the more limited point: the ordering of particular ends can require a form of general evaluative knowledge that is not reducible to the specialized knowledge of each subordinate discipline.
The remaining dispute concerns the nature and explanatory ground of that knowledge.
The practical argument therefore requires an additional premise: that the proposed Platonic knowledge contributes no relevant explanatory, classificatory, or evaluative content beyond what particular sciences and practical wisdom already provide.
That premise cannot be inferred merely from the fact that practitioners require specialized knowledge.
9. The Form of the Good and the Interpretation of Plato
The preceding argument does not establish the existence of a separately subsisting Form.
A common explanatory account might exist without its object possessing independent ontological existence.
The distinction between intelligible unity and ontological separation must therefore remain explicit.
Nevertheless, there are reasons to question whether Plato’s Good should be understood simply as a highest genus.
In the Meno, Socrates seeks the common character in virtue of which different virtues are virtues, rather than an enumeration of their instances (72b–73c).
This passage establishes a methodological distinction between the plurality of things falling under a determination and the common account sought in philosophical inquiry.
It does not itself establish the metaphysics of the Form of the Good.
The more relevant evidence appears in Republic VI–VII.
Through the analogy of the sun, Plato presents the Good as responsible for the intelligibility of objects of knowledge and, in a distinctive sense, for their being (508e–509b).
The Good is not presented merely as another member of a class of good things. It possesses an explanatory priority that complicates its interpretation as an ordinary universal genus.
This supports the possibility of understanding the Good as a principle of intelligibility and being rather than simply a common generic essence.
Such an interpretation remains philosophically and historically contested.
Recent scholarship has emphasized that Aristotle’s criticism targets several distinct commitments often associated with the Platonic Form: universality, univocity, paradigmatic status, and independent existence. These commitments need not stand or fall together.[1]
The present argument does not establish that a non-generic interpretation is the only correct interpretation of Plato.
It identifies a conditional consequence.
If the Platonic Good is understood as a non-generic explanatory ground of differentiated goodness, Aristotle’s objections to generic unity do not directly exclude it.
To establish that exclusion, Aristotle must show either that Plato’s theory necessarily requires the rejected form of univocity or that the proposed non-generic explanation fails on independent grounds.
That is the outstanding burden.
10. The Two Burdens of Proof
The argument can now be stated more precisely.
There are two distinct questions.
First: Has Aristotle established that the relevant Platonic explanatory unity is impossible?
This is the burden of exclusion.
Aristotle’s categorical argument establishes that goodness cannot be one genus common to the categories. His discussion of intrinsic goods raises a stronger challenge to univocity. His arguments from scientific plurality and practical specificity constrain the explanatory and practical roles a universal Good could possess.
But these considerations do not establish that every non-generic explanatory principle is impossible.
Second: Has the Platonist established that a separate Form actually supplies the relevant unity?
This is the burden of explanation.
A Platonist must specify what the proposed Form explains, why that explanation is substantive, and why it cannot be supplied adequately without ontological separation.
The present paper does not discharge this second burden.
Its conclusion is therefore not that Plato’s theory has been demonstrated, but that Aristotle’s objections do not establish its impossibility.
This distinction prevents two symmetrical errors.
The first is to infer the existence of a universal explanatory principle merely from the diversity of things called good.
The second is to infer the impossibility of such a principle merely from that same diversity.
Neither inference is justified without additional argument.
11. Conclusion
Aristotle’s criticism of Plato identifies genuine philosophical difficulties.
Goods belong to different categories, stand in relations of priority and posteriority, possess different proximate explanations, and fall within the competence of different sciences.
Moreover, universal knowledge does not replace the particular expertise required for successful action.
These considerations impose substantial constraints upon any acceptable theory of goodness.
But they do not establish that a common explanatory principle of goodness is impossible.
The categorical argument excludes generic unity without necessarily excluding explanatory unity. The plurality of sciences does not preclude general knowledge of their common structures or objects. Differences among intrinsic goods may establish non-univocity without excluding a higher-order explanation of their differentiated goodness.
And the insufficiency of universal knowledge for particular action does not establish its practical uselessness.
Most importantly, Aristotle himself acknowledges the possibility of non-generic unity through a common source, a common end, or analogy.
His criticism therefore leaves open a question about the explanatory ground of the intelligible relationships among goods.
Could the Platonic Good provide that ground?
The present argument does not establish that it does.
But Aristotle’s objections do not establish that it cannot.
The diversity of goods is not, by itself, an argument against their intelligible unity. To exclude the Platonic explanatory principle, Aristotle must show not merely that goods differ, but that their differences preclude the kind of unity the principle is supposed to explain.
Until that further argument is supplied, the rejection remains incomplete.
Notes
1. The categorical argument and contemporary scholarship. See J. L. Ackrill, “Aristotle on ‘Good’ and the Categories,” in Essays on Plato and Aristotle (Oxford: Clarendon Press, 1997), 201–211; Melina G. Mouzala, “Aristotle’s Criticism of the Platonic Idea of the Good in Nicomachean Ethics 1.6,” Peitho: Examina Antiqua 8, no. 1 (2017): 309–342; and Christopher Shields, Fractured Goodness: Aristotle’s Response to Plato’s Form of the Good (Oxford: Oxford University Press, 2024). Shields provides a systematic recent reconstruction of Aristotle’s objections and their implications for the comparability of goods. The present paper shares some of the critical concerns raised in this literature but concentrates on the narrower question of whether Aristotle’s arguments exclude a non-generic explanatory ground.
2. Priority and posteriority. See Shields, Fractured Goodness, chap. 3, “A Series of Goods.” The argument’s force depends partly upon the restriction Aristotle attributes to the Platonists concerning Forms over ordered series. Compare Eudemian Ethics I.8.
3. Non-generic unity. Aristotle’s discussion of unity through a common source, common end, or analogy occurs at NE I.6, 1096b26–29. He does not settle the issue in this passage. Compare Metaphysics IV.2, 1003a33–b19, on focal unity.
4. Practical usefulness. See Rachel Barney, “The Carpenter and the Good,” in Pursuing the Good: Ethics and Metaphysics in Plato’s Republic, ed. Douglas Cairns, Fritz-Gregor Herrmann, and Terry Penner (Edinburgh: Edinburgh University Press, 2007), 293–319. Barney’s discussion is especially relevant to Aristotle’s challenge concerning the practical contribution of knowledge of the Form.
5. General and particular inquiry. See Dominic Scott, Levels of Argument: A Comparative Study of Plato’s Republic and Aristotle’s Nicomachean Ethics (Oxford: Oxford University Press, 2015), on differences between the methodological ambitions of Plato’s Republic and Aristotle’s Ethics.
6. Intrinsic goods and comparison. See Shields, Fractured Goodness, especially the discussions of univocity and the comparison of goods. The possibility of a common explanatory ground does not by itself resolve how goods are to be ranked; it identifies a possible basis for further inquiry.
7. The Platonic Good. Compare Meno 72b–73c and Republic VI, 508e–509b. The former illustrates inquiry into a common determination; the latter presents the Good as a principle of intelligibility and being. Neither passage alone establishes that the Good must be understood as a genus.
References
Primary texts
Aristotle. Eudemian Ethics. Especially I.8.
Aristotle. Metaphysics. Especially IV.2.
Aristotle. Nicomachean Ethics. Especially I.1–6.
Aristotle. Posterior Analytics. Especially Book I.
Plato. Meno. Especially 72b–73c.
Plato. Republic. Especially Books VI–VII.
Secondary literature
Ackrill, J. L. “Aristotle on ‘Good’ and the Categories.” In Essays on Plato and Aristotle, 201–211. Oxford: Clarendon Press, 1997.
Barney, Rachel. “The Carpenter and the Good.” In Pursuing the Good: Ethics and Metaphysics in Plato’s Republic, edited by Douglas Cairns, Fritz-Gregor Herrmann, and Terry Penner, 293–319. Edinburgh: Edinburgh University Press, 2007.
Mouzala, Melina G. “Aristotle’s Criticism of the Platonic Idea of the Good in Nicomachean Ethics 1.6.” Peitho: Examina Antiqua 8, no. 1 (2017): 309–342.
Scott, Dominic. Levels of Argument: A Comparative Study of Plato’s Republic and Aristotle’s Nicomachean Ethics. Oxford: Oxford University Press, 2015.
Shields, Christopher. Fractured Goodness: Aristotle’s Response to Plato’s Form of the Good. Oxford: Oxford University Press, 2024.
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