Tag Archives: hallucination

Spatial illusions and temporal illusions (II): What Really Surrounds You

Previous: perceiving three dimensions and geometric optical illusions

In my view, a good explanation — or a good philosophical or metaphysical explanation, anyhow — is a general one. It does not just explain one particular thing; it explains a multiple of things. I think there is a limit to any such explanation, due to the particularity of everyday things, though. The thisness — or however you might put it — of any particular object, e.g., this particular orange, means that explanations of how this orange comes about, and/or accounts of its current constitution, cannot be generalised to all oranges, or even a non-singular sub-set of oranges.

A lot of the individuality of that orange, however, can be captured by simply pointing out its unique location in space and in time. The orange is just like other oranges, except it is at this time and this place.[1]

Still, I think the better explanation is the more general one. It is not the firefighting one, the ad-hoc, we-need-it-now/nevermind-then/nevermind-later. It is also not the one which explains only how this orange came to be on my table, but also how oranges come to be on tables in general.

Similarly, if I introduce a concept of space in order to explain a particular geometric-optical illusion, then it is a weakness of my theory that this concept only covers that illusion, doing nothing for other kinds of experience, illusions, hallucinations or otherwise. It is being introduced ad-hoc, and only specifically for that illusion. In contrast, if I introduce a concept of space which is already there, or can be effectively generalised to other things, then there is nothing ad-hoc about this.

So what concepts of space are there already, which might be used to explain geometric-optic illusions?

What Surrounds You?

Let’s start with your idea of space.

If I ask you, what surrounds you, what will you say?

There are two ways I think I might answer this question.

  1. First, I might simply describe what I am aware of, aware of without any obvious reflection or mediation. I just look around me, which is so far as I can tell a simple turning of my neck and keeping open of my eyes, and say: there’s this, and this, and so on.
  2. Second, I might reflect upon it and think about what there has to be and what there can be around me.

‘1’ can be understood as giving a phenomenological description — a description of how things seem. I offer a description even if I am a dying astronaut hallucinating in the depths of a void[2], i.e., where nothing is around me, but there seems to be something.

‘2’ can be understood as doing metaphysics, i.e., formulating a metaphysical theory or speculating about the metaphysical status of the spatial arrangements of things around me. So, for example, if I believe that I am hallucinating, or am under an illusion, this would  affect what I say when asked ‘what surrounds you?’. I would not draw on how things seem to me, since I believe this to be an hallucination or something similar. I might say, in the right circumstances, something like ‘I seem to be surrounded by stars, distant hills, the song of crickets but I am under an illusion’. I’m really surrounded by four illuminated walls in a dark room’.

‘2’ is at least as important as ‘1’ in answering this question. If I answer ‘I’m surrounded by #’ then I don’t mean just what I seem to be surrounded by. I mean what I am surrounded by.

So, with geometric-object illusions, I can describe what I seem to experience and what I think is out there. The illusion is the clash of those: e.g., (from last post) there seem to be uneven lines but the lines are even; there seem to be small circles and big circles, but the circles are all the same size.

So, when I ask you, what surrounds you, what will you say? And in particular, what HAS to surround you? What can’t be surrounding you? And how does this correspond to your experience?

My answer, like many of my answers, is that it depends on how one thinks about time, which requires a lot more detail. For now, we can draw an analogy between the different somewhat live ways of thinking about time and imaginary ways of thinking about space.

Roadrunner: The Painted Tunnel on the Cliffside

Wile E Coyote paints an image of a train tunnel on a sheer cliff side and waits for Roadrunner (and lunch).

Clearly, his plan is: the roadrunner sees what seems to be space stretching into the cliff, tries to run into that space, fails(because there is no space stretching into the cliff), and is knocked out (and becomes lunch),

However, instead: the roadrunner sees what seems to be space stretching into the cliff, runs into that space, does not fail, is not knocked out (and escapes).

Further: Wile E Coyote, puzzled, tries to run after the roadrunner, fails, and is knocked out.

One way to look at the difference here is that roadrunner and the coyote occupy different worlds with different kinds of space in their counterpart areas. In the roadrunner world, space stretches into the cliff; in the coyote world, it doesn’t.

In that case, the apparent spatial extension into the cliff is, in the roadrunner world, not an illusion whereas the apparent spatial extension into the cliff, in the coyote world, it is an illusion.

Something similar can be said of the phenomenology and metaphysics of time.


1. One might be tempted to then say that this is all there is to the ‘thisness’ of any particular orange, or any object. Some metaphysicians do just this; some bundle theorists hold that all there is to an object is the instantiation of properties located at the places and moments in which it exists.

2. For related discussion of hallucination when test pilots are under extreme pressure, see this episode of RadioLab: http://www.radiolab.org/2006/may/05/out-of-body-roger/

Conditions of Illusion II

Back to Conditions of Illusion I.

There are a few other conditions some philosophers think need to be satisfied by illusion, but of which I am less sure. They are that an illusion must (i) be perceptual, (ii) involve external things and (iii) involve properties rather than objects (the mere appearance of which give hallucinations) In this post, I want to briefly discuss the first two of these conditions (I will discuss the last in a later post).

Simple definition

My overall view of what it takes to be an illusion is this:

Illusion (def) =

Ceteris paribus[1], (a) there is an illusion of x if there is (b) an appearance of x and (c) not x.

(Illusion(x) = apparently(x); ~x).

That’s all there is to it. If we have an appearance of something, but not the thing, then we have an illusion. You can argue about what is real (elves? photons? sense-data?), and you can argue about what is apparent (change? Fear? red?)[2]. But once you settle on these, then if you have a discrepancy between these, then you have an illusion. So, my view is also liberal: wherever there is appearance of something being the case without something being the case, be it in perception, cognition, imagination, memory (something I discuss in my 2011b), we have an illusion.

If this definition is sufficient for a definition of illusion, then it is not necessary to go any further in this section. Perhaps you can go back to the main section on illusion, or on to other issues.

However, some might think that this definition of illusion is too simple or too liberal. In the following sections, I briefly examine reasons to think this which I’ve encountered. The point here is to highlight these reasons, and briefly respond, but I don’t pretend here that there is any thorough analysis(at least as I write this post now(15.06.2012); it may change in future, as I think this further through)[3].

Illusions in perception, memory, imagination etc.

I hold that illusions can be further specified by reference to the mode in which an illusion occurs. By mode I mean phenomenological forms of mental activity such as perceptions, imaginings, rememberings, anticipations, volitions, deliberations and whatever else you might hold that there is something you call appearance involved. Thus, one can refer to perceptual illusions, imagination-illusions, memory illusions, volitional illusions, etc. What matters is that, wherever there is phenomenology, wherever something is apparent, there is the possibility of illusion[4]. So, one may have different kinds of illusions, e.g., illusions of memory, imagination, or perception. And as with illusion generally, it may be the case that different metaphysical theories may agree that there is an illusion, but disagree about what kind or mode of illusion there is.

So raises the first objection.

i. Illusion(x) -> perception(x)

I might be mistaken but I don’t think that Guns’n’Roses late ’80s/early ’90s double album Use Your Illusions I and Use Your Illusions II were thinking of ‘illusion’ in a particularly strict sense, requiring phenomenology, metaphysical commitments and so on. I think there is some possibility that they were using illusion to mean ‘false beliefs’. Similarly, someone says they understand something by saying they see it, or the solution to a problem is apparent, and so on; similarly, it seems to some physicist that string theory is a better theory than Loop Quantum Gravity, etc. etc. In that sense, any kind of discrepancy between how things seem — your hunches, your beliefs that you might allow room to be false, but don’t really think they’re false and so on — and how things actually are could be cases of illusion. But there is a more strict sense of illusion, just as there is a more strict sense of appearance, of seeming, seeing and perceiving.

Given this strict sense, one might argue that, contrary to my statement above, illusions only occur in perceptions. There are two reasons I can think for taking this position:

(a) There’s got to be a real thing.

Illusions involve real objects, of which there are discrepancies in the appearance and reality of their properties. I am under an illusion if I see a ball which seems to be, but is not, red. But this is not true of any other mode. This is because the others fail the illusory test in at least one of two ways:

  1. There is no real object: any objects we imagine, remember, anticipate, etc. are not real objects; they are, at best, intentional objects. We only perceive real objects. So, I am not under an illusion if I imagine a ball which seems to be, but is not, red, because, in imagining it, there is no actual or real ball to fail or succeed in having that colour. Similarly, remembered or anticipated balls neither have properties, nor fail to have properties, because they aren’t real.
  2. There is no appearance of a property: some modes do not involve the appearance of a property about which we are mistaken. Thus, there is no illusion.

I think that ‘1’ and ‘2’ can be debated for various reasons, some involving the metaphysics of time (as discussed in my 2011b).  I don’t want to go into detail here but, briefly, contrary to ‘1’, one could argue that other modes than perception can be of real things; not every imagined entity need be unreal; and it is a matter of metaphysical debate that a remembered entity is unreal. And contrary to ‘2’, one could argue that some forms of imagination and memory involve the appearances of what is imagined or remembered (as discussed, for example, by Husserl,  Sartre or McGinn (see bibliography)).

(b) There’s got to be a presented thing

In reply, it might be argued that the imagined or remembered entity, although perhaps real, is always absent(something discussed by Sartre in The Imaginary and McGinn in Mindsight (see bibliography)). But the object of illusion has to be presentor presented to be an illusion. I think that this is a better response than above; imagined or remembered, etc., etc., things are not presented as what we see, hear, etc. (Except I do wonder what exactly it means for a distant star to be presented to you, other than just saying that you see it. Is it that it’s happening now?).

I do not want to restrict illusion this way because I think that there can be a discrepancy between the appearance and reality of what is imagined or remembered, and I think that it is fine to call that an illusion; I think it is fine to talk about memory illusions, for example. One example of imagination illusions discussed in my 2011b is the possible case where a subject seems to be imagining what is in fact present, and not absent; that something is absent is the discrepancy between appearance and reality.

Why should anyone care about this? It matters when we talk about temporal illusions, as I discuss here. It will be reasonable for many  that at least some objects in time are absent objects, and that we can be mistaken about the temporal properties that they seem to have. If we want to talk about temporal illusions, we may need to talk about cases where everything relevant is absent.

The next two issues do not concern the mode of illusion but the content of illusion.

ii. Illusion (x) -> O appears to be x -> external O

Another way of restricting my definition is to insist that the object in the definition not only be real but also be external. By ‘external’ in perceptual theory, I think, one typically means one of two things:

  1. As ‘outside’ the mind, meaning it is mind- (and perhaps phenomenology-) independent.
  2. As ‘outside’ something physical which is related to the mind in an important way, e.g., the brain or body is the physical something, and (a) it is the mind (identity-theory, Smart 1970); it is what realises or correlates with the mind (various other theories, such as epiphenomenalism, functionalism, etc.). Importantly, this physical something is not just anything physical; it has a very specific physical locality such that it is not the world in which a human being moves, with which it interacts, etc.

Typical examples of external things are…well, look around you — what do you see? Tye 2009b (in the company of many others) argues that, typically, all you seem to see are external things (see also Nudds 2009). Chairs, tables, your own hands, the sky, the sea, the floor, the flashes of light in the trees. Similarly with what you hear, taste, feel.

These are external things. The claim is that we are under an illusion only if these external things do not have the properties that they seem to have. It doesn’t matter what else we see, or seem to see, or if anything else does have these properties.

(Assuming here that there are no non-perceptual illusions(see above)) There are three reasons for this:

  1. perceived(O) → external(O): The only objects that we perceive are external things. Given everything we perceive is external, it is always the case that the O which appears to be F is always an external O. This is often called direct realism (e.g., Johnston 2004). It is trivially true, then, that illusions involve illusions about external objects.
  2. O is apparently(F)  apparent (O) -> external (O): Even if we perceive non-external objects (discussed below), there is something special about the perception of external objects which means that we have illusions only in cases where they fail to have the relevant apparent properties. What’s so special about them? Well, as already said: only external objects appear in perception (i.e., Tye’s point above). Thus, they only ever seem to have the relevant properties. And this means that mistakes in this appearance, that they seem to have some property F, but do not have F, are the only cases of illusion of F.
  3. non-external (O) is apparently (F)  ~(O is ~F): Even if we perceive non-external objects, and these objects are apparent, there are no illusions involving these because we are never wrong about the appearance of these internal things’ properties.

I think that none of these three positions are forced upon anyone, unless they already take a metaphysical position in contemporary debates on perception.

Contrary to ‘1’, not everyone thinks that what we perceive must be external. Indirect realism or the indirect theory of perceptionholds that what we directly perceive is internal, not external, e.g., sense-data. So, at least some of what we perceive is internal, and we cannot assume that the object with the illusory property, in all cases, is external.

Further, given how this kind of theory is introduced, some of the apparent properties are properties of the internal entity; we directly perceive those properties, through which we are aware of other (or similar) properties of the external entity. So, for example, sense-data in George Moore’s view are the bearers of the colours and shapes that we directly see.

Contrary to ‘2’, there are these issues: first, why assume that the ‘o’ which seems to be ‘F’ is itself apparent? Why are objects, beyond their properties, apparent to us? This requires a brief digression from discussions about perception into the metaphysics of property-theory.

Some debates about property theory involve debates about the ontological status of substances and particulars. One important question is about the idea that there is anything over and above the properties which we perceive. Russell, for example, is attributed with denying that there is anything over and above properties (and relations), and his thinking has led to the philosophical position of bundle theory. Others argue that properties and relations are not enough; we need substances, substrata or ‘bare particulars’ to have these properties. Then there are others, nominalists or austere nominalists, who reject that there is anything over and above objects; there are not really properties or relations. And still others, often allied with nominalists, who hold that there are properties and relations, but these properties and relations are particular to each object — what they call tropes or abstract particulars (for discussion, see, e.g., Loux 2002; Robinson 2009)

What is the relevance of this debate to questions of illusion? These philosophers seem to generally agree about how things seem. And how things seem is that there are particular coloured, shaped things, located in space and time. The questions are then typically: do we need something extra bearing the properties that we perceive, or is it sufficient that they simply occur together (are compresent), by ‘together’ typically meaning that they occur at the same spatiotemporal location? These questions show an underlying assumption that the appearance of objects is indistinguishable from the appearance of particular instances of their properties occurring together in space and time.

So, if what makes a particular object apparent is instances of its properties bundled together in one place and time, then we can ask: why assume that the apparent object, the object that is apparent to us, is the external object? The answer is: because it seems to be external. However,  it also seems to be red, round, shaped etc. (The internal object has those properties if they are apparent to us; the external object may have it, but it’s not necessary). So why not say that the apparent object is the red object, which might be an internal object, rather than the external object?

E.g., if some thing that we perceive is red, three meters away, and round, and it turns out that we indirectly perceive an external (3m away) thing through directly perceiving an internal red thing, then why assume that the apparent object is the external thing rather than the red thing?

I can’t see any compelling reason for this, once one is no longer a direct realist (in the sense of ‘1’). So I think one can argue that internal things can be apparent to us.

Still, as ‘3’ points out, perhaps we can’t be wrong about the appearance of internal things, so there are no illusions of them.

But: we can be wrong about the appearance of internal things. Note that internal things are introduced as being what we directly perceive. And, flipping the last point: what seems to be directly perceived seems to be external. However, being internal, what we directly perceive is not external. This is the notorious ‘veil of perception’ so disliked by direct and naive realists (see, e.g., Hacker 1987 for arguments from this perspective; Lowe 1981 for a claim that this ‘veil’ is not such a problem). So, there is a discrepancy between the appearance and reality of the properties of these (internal) objects that we directly perceive. Thus, there is an illusion of at least one kind of property had by those objects.

Given how such internal objects are introduced, however, one might still say that, if we have any reason to believe in these things at all, we cannot be under illusions of any of their other properties. This might be right — but it depends on why we introduce the internal objects. It is the case that one motivation for sense-data was epistemological: to be the reliable bearers of direct experience: we could be certain that if we seem to experience a round coloured thing (e.g., the facing side of a red ball), then there is a round coloured thing presented to us (the sense-datum). Olafson 1953 writes that

The same point is sometimes made in a formal way by saying that every sentence in which an object is asserted to appear to have a certain attribute entails another sentence to the effect that something does in fact have that attribute.

(Olafson 1953, p.274)

Given that subjects make true statements about an object appearing to have a certain property, the idea of sense-datum theory, and others like it, is that there is something which has that property. One can assume that at least those properties are real, whatever else we might debate.

As already said, this cannot be true for everything we posit as being apparent. Given the common form of indirect realism is true, the externality of what we directly perceive has to be false. Others find further fault with other kinds of properties being attributed to objects that are not external, e.g., colour and shape, in particular regarding the idea of what exactly is apparent to us, about ‘how things look to us’ (e.g., Overgaard 2010). So, this might mean that any indirect realism theory must be thrown out from the start.

However, I do not think the epistemological reason is the only reason for positing indirect realism. There is also a metaphysical reason: that what we perceive or experience must be real. The metaphysics of time is relevant to this, something long recognized by advocates of sense-datum theory. This is to do with what is real in time.

I think that there are other issues about illusion which I intend to talk about at some point, but I will leave it for now[5].

Next Posts


1. Why ‘all other things being equal’? Arguably, the definition can be complicated by considering an x that is defined relative to something, e.g., x= being gigantic For one subject, A, this might be the case of some O, i.e., for A, O is gigantic; for another subject, B, this might not be the case of some O, i.e., for B, O is not gigantic. If all things were not equal, we could have, for A , (a) apparently(gigantic) but, for B, (b) ~(gigantic), and yet not have an illusion.

2. I consider this question to be either the same question as what is admissible as the contents of experience (e.g., MacPherson 2011b) or a similar question applied to appearances.

3. These reasons come from: (a) Discussions with others, particularly with Fiona MacPherson at the University of Glasgow Philosophy, Petra Vetter at University of Glasgow Psychology, Valterri Arstilla at University of Turku,  Julia Jansen, Lilian O’Brien and Joel Walmsley in University College, Cork, and others over the years. This is not to say that these views belong to any of those I’ve spoken to, however.  (b) Through reflection on what I have encountered in my reading (e.g., definitions of illusion found in Gregory 1987, Parks 1987, Reynolds 1988, Lotto 2003, Smith 2010, Kalderon 2011).

4. However, this does not mean that one must account for the possibility of illusion in one’s account of such modes; just because it is possible for any appearances to be mere appearances does not mean that our actual faculties are capable of this possibility; and so, we should not assume that we need include an explanation of this capability. I consider the view that our faculties are capable of this to be what I call the Principle of Universal Illusory Counterparts (discussed here).

5.  E.g.,

  • Given illusions of one property, should we assume that there can be illusions of any property?
  • Given illusions of one property, should we assume that there can be illusions of some other property?
  • How justified is the claimed difference between hallucination and illusion? Could we just call hallucination a special case of illusion?