On the ten modes of Skepticism

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UPDATE 2017-07-28: For my current analysis of the modes of scepticism, refer to my paper Notes on the modes of Scepticism.


In this essay comments are made on a specific tradition of ancient Skepticism, namely, the ten modes of Aenesidimus of Knossos, which, according to him, lead one to suspend judgement about the validity or falsity of claims on reality. The ten modes, by virtue of the commonalities they share, can be reduced to two: (i) the mode of relation, and (ii) the mode of differentiation. These superordinate tropes are themselves scrutinised in light of a bifurcated conception of reality that hypothesises a distinction between sensible and intelligible objects. Under this scope, analytical stress is pitted on the possible constancy of an idea relative to itself, the two states of objectivity and the regress argument about the criterion of knowledge. The conclusions of such an examination, should they be found plausible, would suggest not a rejection of the skeptical enterprise but could rather inform an inquiry into its basic [pro]positions, ultimately leading to its thoroughgoing reformulation or substitution by a more holistic understanding of first philosophy.

Examining the ten modes

Aenesidimus is commonly referred to as the father of the Pyrrhonian school of skepticism. The name of this tradition is borrowed from Pyrrho of Elis who is thought of as the first to have committed himself to skepticism more thoroughly, consistently and overtly than either his contemporaries or predecessors.

According to its proponents, Pyrrhonism differed from other philosophical tendencies of its time in that it was consistently non-dogmatic. The Pyrrhonian skeptic aimed not at denying the very possibility of certainty, as did for instance the Academic skeptics, but rather at enumerating and stressing the conditions any account on reality would have to satisfy for it to remain open to the truth; a “truth” that the Pyrrhonist would continue to search for, by remaining aporetic (dubitative) and zetetic (inquisitive) about the foundations and final judgements of their given knowledge. Put differently, the intention of the Pyrrhonian skeptic was to form a certain philosophical stance towards reality that would not commit the error of assuming the categories it would seek to vindicate.

In general terms, the Pyrrhonist enterprise can be understood as consisting of two parts: (i) on metaphysics and epistemology taken together (what may be referred to as “first philosophy”) and (ii) on ethics. The ten modes which shall be examined herein fall in the former category. We shall not wrestle with the ethical tenets of Pyrrhonism, for we are here concerned not with the resulting state of affairs of the skeptical method, but rather with whether the modes leading to suspension of judgement, as stipulated by Aenesidimus, do indeed provide for a cogent argument against certainty.

The perhaps most definitive account on Pyrrhonism comes from Sextus Empiricus’ Outlines of Pyrrhonism [1]. In that book we are presented with the ten modes of Aenesidimus (p.27):

The usual tradition amongst older skeptics is that the “modes” by which “suspension” is supposed to be brought about are ten in number; and they also given them synonymous names of “arguments” and “positions”. That are these: the first, based on the variety of animals; the second, on the differences in human beings; the third, on the different structures of the organs of sense; the fourth, on the circumstantial conditions; the fifth, on positions and intervals and locations; the sixth, on intermixtures; the seventh, on the quantities and formations of the underlying objects; the eight, on the fact of relativity; the ninth, on the frequency or rarity of occurrence; the tenth, on the disciplines and customs and laws, the legendary beliefs and the dogmatic convictions. This order, however, we adopt without prejudice.

As superordinate to these there stand three modes—based on the subject who judges, that on the object judged, and that based on both.

The first observation one might make about Aenesidimus’ ten modes is that they are more enumerative than they could have been. The identification of their abstract structure can help us reduce their number to two:

  • the mode of differentiation, which consists of (a) the variety of animals; (b) the differences in human beings; (c) the different structures of the organs of sense;
  • the mode of relation, which covers: (i) the circumstantial conditions; (ii) positions and intervals and locations; (iii) intermixtures; (iv) the quantities and formations of the underlying objects; (v) the fact of relativity; (vi) the frequency or rarity of occurrence; (vii) the disciplines and customs and laws, the legendary beliefs and the dogmatic convictions.

These two higher-order modes, can be recognised as underpinning a set of arguments on: (1) how things differ from and among themselves, and (2) how they differ in a context with other things. The mode of differentiation would thus suggest that a thing can be understood as differing from its perceived original or own kind; while the mode of relation would suggest that a thing can be held different from its perceived original or own kind when it appears in or through a given combination of things and the specific interoperations they have.

Such variability would suggest that genuine regularity is not to be identified and, hence, there is no sound basis for true certainty. If things are changing in themselves and if their relations are not independent of them, any claim about a given instance of perceived connection would have to be limited to said instance and not be elevated to a general regularity of which certainty can be expressed. Positions that do not follow this approach and which therefore rest on dubious grounds, the Pyrrhonist would argue, are but mere opinions that can be juxtaposed with other views of the same epistemic status to demonstrate their incompleteness. Thus, the argument would follow, suspension of judgement on the ultimate [pro]positions would be the only non-opinionated, non-dogmatic approach.

Taking these claims at face value, we may still assume that skepticism would have to exhibit certainty about the very constancy of its skepticism. It would have to affirm that it proceeds with a conduct or method that it identifies and of which it is certain of, for if it were not to, then no Aenesidimus could ever consistently claim to be a skeptic. What is now contended is that concepts such as “skepticism” or “method”, or indeed any of the guiding ideas of the Pyrrhonist do exhibit a constancy of a sort, in not being different from themselves either with or without a context. Our intention is to suggest that the ten modes or the two we made out of them, tend to ignore the magnitude of reality that consists of purely intelligible objects — the thinkable. For, just like Heraclitus and Protagoras who also expounded on theories of differentiation, relativity and total flux, there is no insight as to whether the very ideas of change or rest, of distinction or similarity etc. are also susceptible to these forces or parameters that engender variability.

Let us elaborate on an example to forward the argument and better present the theme now considered: behold a dog. Aenesidimus can furnish all sorts of proof to support the position that no two dogs are alike in terms of their material constitution, such as their DNA sequences never being perfect copies of each other, so that indeed a certain dog is not identical to another one. That holds, he would suggest, not only between dogs of the same breed, but also of the same family, twins even — let alone the apparent differences that exist among dogs of different breeds. In terms of material constitution, and given that DNA structures exhibit differences, it would indeed seem safe to forward the view that each dog is unique and, therefore, no such thing as a definitive “dog” really exists.

Thus, Aenesidimus, by clinging on to the aforementioned ten modes seems to be vindicated, since there appears to be no way of making any positive claim on “dog” when there is no such thing as a constant dog. The object of that study would be elusive. Then the same manner of argument would be applied for the subject of inquiry. What was claimed of “dog”, would be also said of “human” and so on. Hence, Aenesidimus would ask, what is it that we are supposed to affirm when both that which we are about to examine and we who conduct the examination feature no overriding constancy, as we tend to vary in a number of ways? Aren’t these some shaky foundations on which to establish any account on the matter? Are we not then led to suspend judgement rather than rush to support dubious or ill-founded conclusions?

A fair position, one may note, given the factors that would inform a problématique of that sort; and yet, in spite of all such differences, there still remains something that is common in the multitude which allows us — Aenesidimus included — to understand dog as “dog” and not as “bird” or “table” or “philosophical treatise” or the “current president of the European Central Bank” etc. How is it that we can understand that this is a dog, which is somewhat different from that dog, if not by contrasting both of them to something we understand as the midpoint of “dog” or by placing both under a common denominator whatever that may be defined as? By what means, we may further wonder in amazement, can such conflation be avoided when, by meticulously applying the ten modes, we are, in essence, led to blithely conclude that an identity is but an illusion and that in fact all there is, is reasons to think of constancy as a mere fancy?

Instead of providing a direct answer that could give the false impression of arbitrariness and groundlessness, we will opt for outlining a slightly more systematic approach to these concerns. A way of addressing such an issue and of solving the problems of Aenesidimus is by adopting the following line of reasoning which leads to a bifurcated conception of reality as world and as thinkable:

  1. Let us hypothesise that the thought of the thing is not the thing as such;
  2. That would suggest that there is an object in the world and an object in the thinkable;
  3. It can be further assumed that what exists in the world is so independent of its representation, though it is by becoming thinkable that it is conceived as existing and it is only as thinkable that it appears in the way that it does and to the extent of which it does.
  4. Consequently, we may fathom a scenario where what affects one of the two does not necessarily propagate to the other. Therefore, it may be postulated that what has change or the lack of change in itself — what changes or remains regardless of thought — is what originates in the world. I have previously named that element a pragma (plural pragmata) and its class the pragmatic. On the flip-side, what does not have change or the lack of change in itself — what changes or remains because of thought — is what originates in the thinkable. I have previously named that element a chrēma (plural chrēmata) and its class the chrēmatic.

Couched in these terms, let us restate the questions raised above and venture to provide a succinct reply to each of them:

Q1. What is it that permits us to understand dog as “dog” and not as “bird” or “table” or “philosophical treatise” or the “current president of the European Central Bank” etc.?

A1. We seek to trace the simile in multis (that which is common in the multitude), be it with respect to objects of sense or objects of intelligence. With regard to objects of sense, we may suggest that at different orders of constitution there are certain common characteristics, or elements, or arrangements of matter and so on, that are in broad terms similar to one other, featuring more marked differences with other types of constitution of sensible objects. That would however depend on our methodology, of what kind of matter we would be examining and at which order of constitution, since scientists suggest that all of the universe’s entities can be reduced to a relatively small set of elements. On the other side and with respect to objects of intelligence, we can claim that we have an idea of “dog” — an abstraction of all possible dogs we may have ever encountered — which we can relate and compare to any instance of “dog” we may come to identify. Such ideas we also form of other objects, such as “bird”, “table”, “European Central Bank” etc. which we can hold as distinct from each other.

Q2. How is it, furthermore, that we can understand that this is a dog, which is somewhat different from that dog, if not by contrasting both of them to something we understand as the midpoint of “dog” or by placing both under a common denominator whatever that may be defined as?

A2. If there really is nothing we can refer to, we face at the very outset the impossibility of identification. Besides, to draw a comparison we must first be capable of defining its terms. As suggested in the previous answer, we can make an abstraction out of a pattern — the common in the multitude. That abstraction we may use it to understand specific instances as partaking of it or not. Such an abstraction is also made of “dog”, which we consider as supracontextual. When placing in a comparison dog against dog, it is the instances of this abstraction that we consider, not the abstract understanding of “dog”. Any differences are between the instances as against one another, not in the very concept of “dog” in itself; and, a fortiriori, we claim that notwithstanding the manifested differences of these instances, they still belong to their respective class by means of partaking of its idea.

Q3. By what means, we may further wonder in astonishment, can such conflation be avoided when, by meticulously applying the ten modes, we are, in essence, led to blithely conclude that an identity is but an illusion and that in fact all there is, is reasons to think of constancy as a mere fancy?

A3. Having hypothesised that the thought of the thing is not the thing as such; and that what occurs to one may not necessarily propagate to the other, we may concede that identity might indeed not be a property of the object of sense, but of the object of intelligence. What is traced as a constant is an abstract pattern. It is, nonetheless, intelligible and can be made of all things in the world. It is courtesy of such a capacity to recognise the common in the multitude that we may define instances of an abstraction as partaking of it and may, consequently, proceed to compare them to other instances. Conflation is avoided by virtue of us being able of grasping the abstract pattern of each case and of incorporating it into a broader abstract structure we conceive of the world.

To press on with the argument now propounded, it is pertinent to consider the very concept of difference and inquire whether it not only holds constant relative to itself, but also that it retains its form regardless of the constitutive/substantive terms peculiar to any case. 

Consider the following example: it is to be understood that a rectangle differs from a triangle and that a dog is different from a table, so that one may tell a rectangle from a triangle, a dog from a table. In each case, the diligent researcher, let us call him Aenesidimus, may identify the factors/parameters that inform said difference, and may, furthermore acknowledge that between the comparisons of rectangle-triangle and dog-table, the terms of distinction are not of the same sort. Aenesidimus would indeed be in a position to claim that the way in which a rectangle can be discerned from a triangle bears no qualitative resemblance to the way a dog compares to a table. Correct as that observation would be, Aenesidimus would still be labouring under a uniform, formal understanding of difference; for in spite of such disparities between the two comparisons, a common conclusion would be drawn, namely: that the things examined and found as different from each other in ways that are dissimilar, do share the common characteristic of both being conceived as different. Thus, notwithstanding the substantive terms of the comparisons — the constitution of each case — difference would be understood qua difference.

Consequently, if we are to identify the common in the multitude of these two otherwise distinct cases, we may indeed acknowledge that they are conceived as partaking of the abstraction of difference; their commonality in sharing the characteristic of difference standing at the formal, not the constitutional/substantive level. Again, as with “dog” and the instances of it, the gist of the matter is that the idea can be conceived as identical to itself regardless of the particular terms.

Admittedly, the aforementioned raise the following question: does the identification of the abstract pattern provide foundations for certainty? To deal with this issue, the present author finds it necessary that we consider the notion of “objectivity” and then inquire upon which foundations are necessary for knowledge.

To identify constancy one would need to make use of a criterion, by which it is meant to utilise “something”, whose constancy they are certain of, as a benchmark for examining the state of other things. Aenesidimus’ contention with his ten modes is that human cannot ever be said criterion, basically due to its differentiability (mode of differentiation) and the fact that any inquiry would inevitably be conditioned by the mode of relation. Thus, his conclusion would be that the criterion itself is not genuinely objective. To address this problem, it may first be held that regardless of what certainty and knowledge may be defined as and of how they may be attained, they would always have to provide for the discernment of it from them. Without any means of distinguishing between objects, no classification would be possible, no conceptual order, no distinction between a set, a class and an element, no “ten modes of skepticism” etc. In short, no account on anything could ever be proposed in the complete absence of clarity of concept and precision of statement.

Our starting point, if we assume as valid the hypothetical distinction between the world and the thinkable, has to be the very capacity human has of distinguishing between sensible and intelligible objects. Whatever may apply to objects of sense does not necessarily propagate to objects of intelligence. A tendency such as change may indeed appear as overriding the things in the world, yet it does not necessarily ramify to the thinkable, since, as discussed above, an idea can be held constant. The criterion for appreciating the ‘nature’ of such ideas is human. Aenesidimus would suggest that here lies a cyclicality of argumentation and, hence, it must be ruled out if one is to remain undogmatic. On the face of it, we could not agree with that view for the very reason that it would also have to be underpinned by a certainty of a sort — Aenesidimus could not consistently doubt that he would be doubting. Furthermore, to agree with his objection we would have to share his understanding of objectivity as consisting of only one state. Alas, we do not, hence we digress to write about proximate and ultimate states of objectivity.  

That a criterion of a sort is used to grasp the way certain things are, does not invalidate any means of discernment gained out of it. With human as criterion, an acquaintance with things is achieved and, with the application of the proper method, regularities may be discovered, examined and categorised. There is some knowledge derived from the application of this criterion, both with respect to sensible and intelligible objects. With regard to intelligible objects, constancy is found in the abstraction being identical to itself. As for objects of sense, there are regularities to be identified in the manner of constitution of things, the way they complement or react to one another etc. These may exhibit no identities whatsoever, but can manifest resemblances and commonalities. 

The very possibility of presenting this argument is an indication of some certainty underpinning conduct and thought. The text has a given structure and discusses some themes, all of which are conceived independently of others that are not considered herein and, a fortiriori, are (hopefully) not confounded with them, at least not in a strict interpretation of concepts. Furthermore, the present author makes use of a definite number of technological instruments that enable the typing, formatting, presenting and publishing of this very essay. These tools are hardware and software that was developed with a degree of precision and with the intention of functioning in a specific way or range thereof. The level of certainty characterising the argument qua argument and the technology now in use may vary, yet their commonality consists in none of them being products of arbitrariness or uncertainty — an indication that ‘some kind’ of certainty can be discerned.

May the scenario be fathomed that such certainty eventually informs a complete account of the world? If that were so, would it completely invalidate any skeptical [pro]position? Perhaps, it could address all of them bar one: the regress argument concerning the criterion. Aenesidimus would ultimately claim that in having singled out a criterion we have essentially been trapped in a circular argument, as all we do is furnish as proof positions that need further positions to be justified — and we can do so ad infinitum. There would be no transcendental foundations to our argument, so that our claims would ultimately be self-referential. That may be so, though it would have to concern not the proximate state of objectivity (and of certainty) but the ultimate one. In doing science and philosophy we would continue to be guided by some certainty, but maybe not by one stemming from foundations outside our criterion. Aenesidimus would continue to propound his skepticism with the certainty of it being constant, though he would not be in a position to be certain of whether his conduct was or could ever be established on something other than the human criterion.

To recapitulate, if we were to remain true to the effort of not being dogmatic, we would find it useful to posit that ‘some’ certainty can be found, even though it may not be definitive; hence, the suggestion of assuming that objectivity consists of proximate and ultimate states.


Aenesidimus forwards his argument by enumerating the factors of irregularity that characterise the world. These he calls them modes or positions that inform — or lead to — a skeptical approach towards claims on reality. The aim is to demonstrate the absence of constancy and, if successful in that endeavour, to conclude that no certainty can be predicated on such dubious foundations. The present author would tend to find the core idea of Aenesidimus a plausible one, for, admittedly, what underlies or permeates the discussion thus far is the assumption that arbitrariness cannot be sufficient grounds for certainty and, conversely, that knowledge rests on some sort of constancy or regularity.

Our departure from Aenesidimus’ position commences at the point where we hypothesise a distinction between the world and the thinkable and proceed to theorise about epistemics thenceforth. What is at stake, what the ten modes seem to neglect, is the possibility of recognising an intelligible sign that permits identification and, hence, that prevents total conflation. Aenesidimus seems to expound on the variability of the world, while disregarding the realm of intelligible objects. Hence, his argument against certainty remains, at the very best, contained to an aspect of the broader view. Having employed a different method than that of the ancient skeptic, we have been led to conclusions that differ from his. A general final note on such an exercise could therefore be that our manner of approaching the subject can both reveal and obfuscate that which is examined.


  1. Empiricus, S. (1990). Outlines of Pyrrhonism. Prometheus Books, Amherst, New York