Tag Archives: possibility

What illusions do theories of perception have to explain?

Back: Principle of Universal Illusory Counterpart

[You can see this post as connecting to and overlapping a little with my 2018 book‘s chapters on different kinds of experiential error and choosing between them. But this is about something else, whether or not we should explain merely possible illusions or not (I think not) and what that means for theories of perception.]


I show you an image and ask you ‘what do you see?’

You answer that you see, e.g., ‘such-and-such x’, some kind K, property P, relation R, or a combination of similar things — particulars, kinds, properties, relations.

And I answer: ‘ok. But there are no K, P, R, x, or combination of these or things similar. This is an illusion. ‘
In response, you say ‘wow…’ (Because you would) and then ‘but how can there be this illusion?’

What should I say?

  • I might say ‘because how you see can lead to illusions and this is one of them.’ However, this is not an answer involving new information. If I’m assuming that the illusion comes from my seeing, I already know that seeing leads to illusions, and this is one of them.
  • So, I might explain how perception works. In so doing, I explain how the way perception works can sometimes result in the illusion I demonstrate here. That is, my theory of perception explains this instance of illusion.[1]


Generally, what cases of illusion should I be explaining here? Need I only explain illusions that are actual or must I also explain illusions that are merely possible? In this section, I discuss why I think we need only explain actual illusions. This is not to say we cannot explain merely possible illusions, but that our theories ought only to explain the actual ones. (The section to follow, on what actual illusions show, discusses what we can predict from such actual illusions as regards possible illusions).

Universal Illusion

Like Uncle Petros dreaming the solution to Goldbach’s Conjecture, what might be apparent could extend very far, and fail to correspond to how things are. As said before, some might use ‘appearance’ to correspond to what one believes, and this can include mathematical claims, and beliefs about abstract entities such as universals, unobtained possible world, etc. There are also those who think that some ways of understanding such abstract entities are less like some general belief and much closer to the perception of the world, e.g., Godel thought his understanding of maths was a form of perception, a perception of Platonic mathematical entities (Yourgrau 2005)).

Here, I want to narrow the appearance of a mind-independent world, and that it is only a dream, to concrete particulars and their exemplification of properties, relations, etc. With that, I think we can say the following:

  • Seemingly, I perceive and remember particular spatially, temporally located entities and the properties that they possess. Consistent with such an appearance, or such phenomenology, is that (for perception) there are no and (for memory) there never were any of these particular spatially, temporally located entities and properties.
  • If there are or never were such spatially, temporally located entities, my entire experience is an hallucination.
  • If there are (and were) particular entities, but they have none of the properties that I seem to perceive, my entire experience is an illusion.

Illusions will only concern me, for now (I’ll put aside the further complexity of hallucinations  for a later post).

Thus, the possibility concerns only the properties that I seem to perceive. With that, then, we say this:

It is possible that every property which I seem perceive or seem to have ever perceived is merely apparent. There is no and never was such a property (at least, properly connected to me via perception and memory — but take that as implied in what I say hereon). Let us call this possibility the possibility of Universal Illusion: ‘illusion’ because there is the appearance of a property without there being a real property; ‘universal’ because it applies to all properties which one seems to experience or to have experienced.

The question then is this: Does one have to accommodate the possibility of Universal Illusion in one’s theory of perception?

We might ask the question this way: When one explains how one’s perception works, including how certain appearances arise, and how they connect to the external world, must one include an account which allows that any appearance of a property can occur without there being such a property?

Or we might ask it this way: In providing an analysis of perception as a set of features,  including phenomenology, causation, and conditions for how it actually occurs,  must one include in that analysis, have it built into the system, the possibility to generate universal illusion — even if, as one assumes of one’s actual perception, no such universal illusion ever occurs?

In answer to all of these, I would say: we need not cover the possibility of universal illusion in our perceptual theories.

Actual capacity

Something, such as universal illusion, may be possible, but if it is only merely possible, one does not have to include in what is actual. How our perception — yours, mine — works is something actual; the range of appearances it can give rise to is an actual range, not a merely possible range. The merely possible is useful when developing accounts of this range. But nothing merely possible is necessary for it. So, for any appearance of x, it may be possible that there is no real x. But, if in providing a theory of perception, I do not accommodate that possibility, with no other motivation to do so (see below) then this is not a failure of my theory. If one presents a perceptual system which does not give rise to universal illusion, one does not fail to provide a suitable perceptual system for our actual perception.

Why would you chose a perceptual system which does not have the capacity for any illusion over one that does? I will discuss this in detail in another post (on the importance of appearances for empirical theory). But briefly, here, my view is this:

  • The positing of illusion is either irrelevant to a theory, where the theory has no empirical content, or is a bad-making feature of a theory with empirical content: the more one’s theory relegates appearances to illusions, the worse it is as an empirical theory. This is to do with the role appearances play in an empirical theory.
  • So, perhaps we have a choice between (a) a perceptual theory which gives rise to any illusion or (b) a perceptual theory which restricts what kinds of illusion can arise. If one is free to chose, for the empirical reasons above, one should select ‘b’: it limits the number of illusions by limiting the number of illusions that can arise; one can say, given (b), in certain cases of appearance, there is no illusion here because there is never illusion here.

More is to be said on this. Here, I assume that a theory of perception need not provide an account involving the capacity for universal illusion, and, even more, the less illusions it is committed to the better. If one thinks we need perception to have the capacity for universal illusion, one needs more than the mere possibility of universal illusion. One needs something to push us in this direction.

Actual illusions

One would fail to provide a good perceptual theory if one did not account for actual cases of illusion. In such a case, there are actual cases of perceptual illusion which are not accounted for by your perceptual theory. Such actual perceptual illusions are cases of a certain relationship between perceptual experience and the world otherwise (in the case of illusion, the relationship is at least a discrepancy between perceptual appearance and the world).

This leads to my next assumption about theories of perception: perceptual theories need not be about perception in a void; they need only be theories about perception in the world (they can, of course, be consistent with a theory of perception in a void). This means, first, a perceptual theory which does not account for the actual range of relationships is a perceptual theory fails to account for something actual. If one element in that range is an illusion, the theory fails to account for an actual illusion.

But it also means that, should a theory require that, in some cases, perception is constituted by worldly entities, things which are impossible given perception in a void, this is not a weakness of the theory. I assume that it is not a condition of a perceptual theory that it be consistent with Descartes’ imagined scenario of an evil demon, nor with a solipsist’s “world”(if it can be called that).

This suggests the following: one’s perceptual theory need only be worldly; further, they have an empirical aspect to them concerning actual empirical evidence. It is tested by actual illusions, not merely possible ones.

This is not exhaustive of what perceptual theories have to explain. But it is, I think, an important part of what must be uncontroversially explained. A reductive or eliminativist physicalist might need to reduce or eliminate  ‘perception’ itself and, in so doing, explain why we refer to it. But this is something which other theories will not be concerned with.


1. I might also take back what I said and explain that in fact there is no illusion here. There is no discrepancy between how things seem and how things are. There is a discrepancy between how things are and something other than how things seem. This ‘other’ is what makes me believe or want to report that I see K, P, R, x, a combination of these, etc.


Universal Illusory Counterparts

Previous: Conditions of Illusion II

Ought we hold the following as a principle about experience?

For any entity or property x that can be apparent, in cases where there is no x, our perceptual (and, more broadly, experiential) capacities include the ability to generate appearances of x.

Here is either the same idea put in a different way or an ever-so-slightly different idea:

Due to how actual experience is constituted, it is possible that, for any appearance of x, x’s appearance can occur without x. How actual experience is constituted includes the capacity, for any apparent x, that there is an illusion of x.

Here is one more way of putting it.

For every case where appearances and reality match, then given our experiential capacities, it is possible for there to be a counterpart case where appearances and reality do not match.

(I think it is plausible that) It follows from holding this (or any of these, if they are different) as a principle that, for every veridical experience, every case where how things appear is how things are, there can be a counterpart non-veridical experience where how things appearis not how things are.[1]

Expressed in the last way, I call this the Principle of Universal Illusory Counterparts.

Dallas Pam

Seeking an answer to the sceptics’ challenge to the dogmatists, Descartes decides to put aside all beliefs about which he had reason to doubt. In Meditation 1, he begins with beliefs derived from the senses:

“All that I have, up to this moment, accepted as possessed of the highest truth and certainty, I received either from or through the senses. I observed, however, that these sometimes misled us; and it is the part of prudence not to place absolute confidence in that by which we have even once been deceived.”

Descartes then moves on to the possibility that everything, no matter how certain it seems, is not how things are. This includes the certainty that one is awake: he allows the possibility that, even as one is certain one is awake, it is the case that one is dreaming.

Let us assume that this is possible: your whole life is, and has been, a dream. Everything that you seem to remember, and everything you seem to see, hear, and otherwise perceive, is not how things are. Or, perhaps, let’s not even go this far. Let’s just go back to your teenage years (if you’re an adult) or childhood (if you’re a teenager): some night, you went to sleep and dreamed everything that appears to have happened to you since then. You are still in that night, dreaming, much younger than it seems to you that you are. At some point in the future, maybe before the end of this sentence, maybe much further in this dreamed life, you will wake up. This life you believe you have lived has no existence. All the people around you are fictions. When you wake up, it will stay with you as any dream does, perhaps make you feel — briefly– sad, amazed, uncertain, relieved, but like all dreams it will fade. Like most dreams I’ve ever had, you’ll forget the details soon enough and simply say ‘I had such a strange dream. I was so much older than I am now, and this happened — and this — and this’, but then you will get on with your — so it seems to you — real waking life.

If the above is possible, then this is also possible: The vivacious, rich, detailed experience in perception (and remembering) is entirely mistaken. It seems to be of a rich, mind-independent world, full of  spatial and temporal entities possessing mind-independent and even body-independent properties such as shape, timbre, colour, weight; but this experience is not of those entities and properties. Even if there are such entities and properties, you have no experience of them. What seems to you to be perceptions or memories are not connected in the ‘right way’ to them (e.g., through causal chains, Coates 2000).

I do not want to dispute this possibility. To put it in terms of possible worlds, I will assume like many that: there is a possible world which seems exactly the same as the actual one but which is completely different to the actual world on the macroscopic and otherwise observable scale.(Although Bouiswama 1949 has an interesting way of resisting this idea.)[2]

However, I do want to dispute that there is a need to accommodate such a world in our explanations of our own perception and experience — that is, of the perceptual and experiential systems that we ourselves, actual beings in this world, possess.

Next Posts


1. This is not to say that, for any particular x, this ever happens to either ourselves or any other actual individual being that has experience. It is only to say that it is possible, given any such actual beings’ capacities. The range of possible ways actual experience can occur, given how it actually works, includes a separation of appearance and reality for any x (which is I suppose another way of putting this).

2. Putnam discusses the relevances of differences on the microscopic scale to psychological states, in terms of their reference/content/meaning, and truth-value. Perhaps one could also run something similar on the cosmological scale, so that if a subject ended up, unknown to themselves, passing through a multiversal wormhole into a universe in which the galaxies are not drifting apart, or where the sun is the centre of the universe, they would mean a different thing when they talk about the sun thinking, or the milky way galaxy glowing beautifully at night. But, in any case, I don’t think that water happening to be H2O or the universe being properly described according to a sort of neo-Ptolemaic system would make our experience of the world mistaken or illusory in the sense intended here.