"Thus all objects appearing to us are seen as being in some place or other, at a certain distance, and in a certain position, and each of these factors makes a great difference in the sense-impressions, as we have mentioned. Hence, by this mode also we shall be compelled to have recourse to suspension of judgement. In point of fact, anyone who wishes to give any of these sense-impressions the preference over the others will be undertaking an impossible task. For if he makes his judgement simply and solely without proof, he will be discredited; and if he wishes to employ a proof, then says his proof is false, he will be refuting himself; if he says the proof is true, he will be asked for a proof of its truth, and another proof for that one, and so on ad infinitum." (Outlines of Pyrrhonism, The Fifth Mode)

Sextus' claims seem simple but yet somewhat counter-intuitive at first sight, but invoke numerous interesting questions upon closer inspection. There is plenty of room for disagreement over what exactly is being appealed to and what conclusions are hoped to be drawn. The two issues raised seem to be the inconstancy of sense impression and how attempts to determine criteria for which sense impressions to prefer as correct will always be arbitrary. There is a misuse of the term 'proof' here as something proven can be taken as true, but what is really referred to is that criteria of truth for sensory impressions must have good reason, but if we give reasons we may in turn expect to have to justify those reasons, and so on ad infinitum.

Sextus can be interpreted to be making a valid and an insightful point. To fully explain this claim I will outline how we do come to regular and constant descriptions of the world and how this relates to the inconstant sensory impressions Sextus refers to. I will suggest that Sextus' actual point does not deny that we can have constant descriptions, but also that there is still room for a sceptical conclusion to be made. This will allow a reasonable and non-counter-intuitive interpretation of Sextus' argument.

Sextus is correct that our sensory impressions are inconstant and variable. If we imagine any object we can imagine numerous different possible sensory impressions of it. Berkeley notes how our sensory impressions of the warmth of an object will often vary, for instance; "Suppose now one of your hands hot, and the other cold, and that they are both at once put into the same vessel of water, in an intermediate state; will not the water seem cold to one hand, and warm to the other?" (Berkeley 1988: 128). Warmth then would be an example of a sensory quality that is inconstant and variable. Warmth, however, is a 'secondary quality', meaning a quality that does not exist in things themselves and thus can be safely claimed to be subjective. Sextus wishes to apply the same argument to size and shape, or, otherwise put, to extension. Extension is a 'primary quality' which is held to be independent of us. This is a much more radical claim as we can't dismiss primary qualities as subjective.

Sextus makes multiple arguments in the fifth mode based on how things seem, some of which are very weak. That Amber hardens in the air is not remarkable, neither is how different notes are produced when channelling air through differently sized holes. Both of these facts do not dispute that objective qualities exist in objects and are constant. These arguments can thus be dismissed.

The better arguments appeal to subjectivity in extension itself. For instance, that "the same boat appears small and stationary from a distance, and large and moving from close by" (Sextus 1985: 63), and that a portico will look a different shape depending on the angle we look at it, "seen from either end, it appears tapered … viewed from the middle appears symmetrical" (Sextus 1985: 63). An attack of this sort attacks the constancy of primary qualities, as it is an attack on both motion and extension (the former being related to extension so far that an attack on the latter is an attack on the former anyway). Similar arguments to the former are made by Berkeley when he writes; "what you can hardly discern, will to another extremely minute animal appears as some huge mountain" (Berkeley 1988: 138), noting how our perception of size is determined by our size relative to the object, and hence subjective (not objective as it should be). The latter argument has similarities to observations by Kant when he notes that sense impressions don't themselves provide us with knowledge of an objects three-dimensional extension, but rather that our minds have to read that into our sense impression as a 'form of all phenomena' (Kant: 2002: 34) which is not learnt via the senses. We never truly see an image of a three-dimensional object, such as a cube, as some of its figure or sides will be hidden from view.

As to the argument noting that objects take up different amounts of space on the visual field, we have no way of assenting that one degree of space occupied is correct and another incorrect. It is impossible to say whether we see things at their correct length at five or ten yards distance. This argument holds, as far as sensible length is concerned (by which I mean length as represented by space taken up on the visual field). There is no reason to suggest that one sensible length of an object is more correct than another.

The argument concerned with perception of space is more complex. A cube from one angle might look only like a square, or as composed of multiple sides but never all six. At no one point could we ever be immediately sensible (aware via the senses) of all six sides of the cube. We might claim that we have no right to knowledge of the other side of the cube, but we would normally have no fear of asserting that it is still a full cube despite not seeing all six sides at any one moment. The problem here is that we have been treating our perception of objects as being composed of single images, not a collection of images.

Taking sensory images one at a time creates the problem Sextus seems to present for us. Without at least two images we should also have no claim to know anything about depth at all as a single image lacks depth. However, this can be solved by realising that we experience the world as a chain of pictures, not individual images.

Sensory experience considered as a series of experiences can make room for some kind of constant description of the world. In illustration, I shall use the descriptor for length constants called 'the inch'. When we describe an object as being an inch long we are not describing the amount of space it takes up on the visual field as an inch long object may take up no or all of the visual field. The 'inch' has no sensible length in that sense of the term. It does describe something constant however; two inch long objects placed side by side will always appear the same length, and they will increase uniformly and consistently in size the closer to the eye they are. The inch does not measure our sensory impression of size, but a regularity among our sensory impressions of size. It is a measurement abstracted from multiple sensory impressions.

Similarly to the inch, our notions of three-dimensional shapes are not based on single images but are abstracted from multiple sensory images. In the sense that Sextus appeals to we have never seen a 'cube', nor could we ever visualise one as we could never imagine an image containing all six sides of a cube at once. The cube then must only be imagined in an abstracted and non-visual way. The importance of this fact is that our constant descriptions of the world are not found in sensory impressions as such, but extracted from groups of them, and that's why Sextus could not find constancy in sensory impressions themselves when he discusses the fifth mode.

This conclusion could be ungenerously interpreted as a knockdown refutation of Sextus' Fifth Mode. If we take Sextus to be suggesting, via the fifth mode, that we have no reasonable shared and constant means of measurements of extension then he is clearly wrong. As physics relies on these measurement constants I refer to and Sextus writes that Sceptics "do not theorize about physics in order to give firm and confident opinions on any of the things in physical theory about which firm doctrines are held." (Sextus 1985: 37) Sextus may be read as implying that we can come to no sure claims about the kinds of regularities amongst our sensory impressions that I hold abstract measurements to describe, that despite everyone's shared observation of the 'inch' in nature everywhere and at all time that we can not be sure of it. In this case, Sextus can be interpreted to be saying something unreasonable and counter-intuitive.

However, if we want to abide by the principle of charity and interpret Sextus in a more generous way, we could suggest his main conclusions are other than the above. I draw attention to Sextus' declaration that that "We do not deny those things which .. lead us involuntarily to give our assent to them" (Sextus 1985: 38). The obvious thing that Sextus includes in this category are sense impressions themselves, that I see an image or feel a warmth is something I cannot deny to myself. There are however other things which our minds have no choice but to assent to.

By our innate natures and psychologies, our sensory impressions are organised by our minds into a three-dimensional spatial world picture, that is a world picture that includes the notion of objective extension which can be measured by abstract measurements like the inch, and hence our objective descriptions of the world. This feature of our mind is very similar to the way David Hume describes induction where no reasoning supports inductive inferences but that "there is no danger that the reasoning, on which almost all knowledge depends, will ever be affected by such a discovery" (Hume 1992: 41) due to it being secured "instinct or mechanical tendency". Indeed, our organisation of sensory impressions into this kind of world picture is more fundamental, it is the 'form of phenomena' itself to borrow a term from Kant (Kant: 2002: 34), and not only can we not stop ourselves from doing it, or assenting to it, we can't conceive of doing otherwise. This means that Sextus' sceptical philosophy allows for us to affirm something is an inch because we cannot choose to not assent.

Sextus, however, is not a dogmatist and it would be absurd to suggest that we can interpret Sextus as arguing that we can have a full certainty about the world, so I shall need to show how his argument still results in scepticism. The sceptical doubt should not belong to our measurements of the world, but the nature of the world, that is, of 'extension' itself.

The objective qualities of 'extension' are indeed objective, but they aren't qualities we can form any clear idea of. As mentioned, they are abstracted from sensory data, so we cannot form an idea of them from our sight or other sensations, especially noting that extension is constant while all our sensory impressions are inconstant. Just like the feeling of warmth is distinct from temperature, our images of things are distinct from extension. This has to be true as all sensory impressions require a mind to impress upon, and thus are mind-dependant, whilst extension exists outside of our minds.

This leaves us with a big question that still needs answering, that question is 'What is Extension?' Scientists make reference to the concept of 'matter', but it is an empty a concept upon analysis. We cannot talk intelligently about what matter actually is of itself, for if we ever found what it was composed of we would simply have discovered that what we take as matter is really composed of something more basic, and that more basic component would then be what really is 'matter'. There is also no way of imagining how this 'matter' could somehow evoke, by unknown process, immaterial sensations in our minds. Berkeley's solution is that all of our objects are just abstract ideas held in the mind of God, or "external archetype[s] on my principles external … though indeed it must be supposed to exist in that Mind which comprehends all things" (Berkeley 1988: 194) though we would be in just as much doubt about how God communicates these sensations into our minds as we would be with matter.

The concept of matter is still useful regardless as we can use it to create a constant and reliable world picture that allows us to interact with reality, and we do this involentarily, and thus would not be doubted by Sextus. However, we do not have an involentary notion of what matter is, or to it's nature and substance (it may not even make sense to talk of the 'substance' of 'matter' as the two terms may be synonymous) nor do we know how these 'matter' creates our sensory impressions, and it may be possible that we could never know these things. This is where we should be skeptical and withold judgement on what we cannot know. This is not counter-intuitive, it is indeed what most people not involved with philosophy do. It seems we should do as Sextus says and not create firm opinions on the matter, and thus by this interpretation Sextus is correct.

Sextus, (1985) Scepticism in P. Hallie (ed) Selections from the Major Writings on Scepticism, Man, & God trans. S. Etheridge, Indianapolis: Hackett Publshng Co

Berkeley, G. (1988) Three Dialogues in R. Woolhouse (ed) Principles of Human Knowledge and Three Dialogues, London: Penguin Books,

Kant, I. (2002) The Critique of Pure Reason, trans. F. Haywood, London: Living Time Press

Hume, D (1992) Enquires Concerning Human Understanding And Concerning The Principles of Morality, New York: Oxford University Press

Despite the above consisting of merely the work of a graduate, nor a post-graduate nor any professional academic, I felt it was important to provide proper referencing information for anyone who might possibly require it.

I do this partly because I used to find it annoying to read pages online and struggle to find the information I required in order to reference the article, and partly because plagiarism is a very serious issue in many educational establishments.

Hence for the sake of the possibility of someone needing it, here would be a typical format for referencing this article. You should check with your university department in order to get precise instructions of how to format your bibliography.

Hankins, J. (2007) Sceptical Doubts Regarding 'Substance', Available from the World Wide Web: http://www.skeletalroses.co.uk/html/articles/metaphysics_scepticism_of_substance.htm