I guess I'm more looking for clarity here than anything else. I've been reading about the illusions that stage magicians perform, and also found web sites containing optical illusions, but I haven't seen much real explanation about how these illusions work at a more fundamental level. I don't really mean how they are made, but perhaps the more philosophical questions about how they are possible, and given that this possibility, what this means about the relationship between our perceptions and reality.

First, I have a terminological confusion. I want to say that illusions aren't really about our senses, but are about our intuitions. But the SEP article on intuition wants to define intuition as a kind of belief or reflective judgment, such as our moral or logical intuition. But when you watch a stage magician fly through the air, this isn't really an intuition in this sense. The Wikipedia article on intuition (psychology) wants to discuss intuition as, for instance, our ability to perceive other people's emotions. Again, this isn't really what I mean. Is there a better word for this, or is it legitimate to tact on another meaning?

I want to say that when I watch a magician fly through the air, my perception is correct, but because I am seeing him but not the wires holding him up, my intuition is giving me the illusion that he is flying. I don't want to call this an inference, as I think inferences are more deliberate, but my intuition that he is actually flying is more immediate than any sort of judgment.

Part of the reason I want a word like "intuition" is because "illusion", in many cases, goes too far. That the magician flies through the air is both an illusion and an intuition, but a drawing of a cube on a paper, where a flat drawing looks three-dimensional, even though there is no way I could be deceived by the drawing. Again, "intuition" in this sense isn't a mistake in my perception, yet is also more immediate than an inference. Please tell me there is a term for this.

Looking at the various optical illusions available online (just Google for it for examples) I see many illusions that seem to indicate the following things:

  • Our perceptual apparatus seems to be too sensitive towards motion.See examples here. It seems easy for us to sense motion where it doesn't exist. Again, this is both used for illusions (to deceive or effect) as well as for non-illusions: video basically causes us to see motion that isn't there. Pixels turning on and off in certain patterns causes us to see motion that isn't really there. This begs the question of whether motion itself could be purely an intuition. How would we know whether motion exists in itself?

  • We seem really bad at making visual comparisons. For example. The apparent size of an object can change depending on the size of objects around it. This, in my opinion, should question whether size and magnitude are pure intuitions and don't exist in themselves. You could just say that we can be deceived by just visually assessing size, but if you measure an object using the proper method, you will get an accurate measurement. But to me the illusion shows me that size is an intuition, and it would violate Occam's Razor to say that size is both an intuition and exists in itself, when it is simpler (it posits fewer entities) to say that it exists only in the intuition.

I'm not going to go on. But the more I think about this problem, the more I worry that our intuitions might compose most, if not all, of our understanding of reality. A good example of a non-optical illusion would be the way we hear voices. I have heard my name being called before when it was only the wind, so why should I believe that someone is actually speaking to me if it is really my intuition that is putting words the noises?

Lastly, if our immediate understanding of the world is wholly determined by our intuitions, and nothing of our phenomena exists in itself, how do we distinguish reality from appearances? If we say that the magician isn't really flying, even though our intuition says he is flying, then our intuitions can be wrong. So how do we determine if an intuition is true or false? If I measure an apple with a ruler, why should I trust this intuition, but not the flying magician? It seems that there are several distinctions that need to be made here, but I think we should be choosy on finding the right ones. A good answer would avoid any ad hoc hypotheses, as my question isn't about any particular illusion, but about the phenomenon at large. A good answer may also give me a good reference if this topic has already been discussed in depth.

  • This seems to be more a of a cogsci question. A friend of mine did at least part of his PhD by looking at how various illusions in perception tell us about what's going on upstairs. An example (not by my friend): Grossberg 1999 The Link between Brain Learning, Attention, and Consciousness.
    – labreuer
    Jan 15, 2014 at 20:32
  • That's a way too extensive and nuanced topic to discuss in the form of a single Q&A, I believe, but an answer I wrote a while ego on a question about (in)fallibility of intuitions might be helpful as I try to elucidate the nature and different categories of intuition which I think is crucial to your problem which I see it mainly stemming from a confusion with different types of intuition and their interaction with sensual perceptions.
    – infatuated
    Nov 20, 2014 at 8:20

4 Answers 4


The Munker Illusion: Science and Philosophy by Jack Schwartz (me) http://bioperipatetic.com/the-munker-illusion-science-and-philosophy/

The Munker Illusion is offered as yet another example of a so-called optical ‘illusion’. Illusions have throughout history been offered as proof that we cannot trust our senses and that all ideas that depend upon sense-perception are therefore unreliable, leading to absolute skepticism, solipsism, and subjectivism, ideas that dominate modern philosophy.

The interpretation of the Munker effect as an instance of perceptual illusion, is the consequence of failing to grasp the true nature of perception, the nature of what our senses are ‘designed’ to detect and process, and attempting instead to rewrite the laws of perception to meet the philosophical demands for ‘valid’ contact with the ‘real world’, typically implying unmediated or unprocessed interaction with the real properties of the physical world. A typical example of such a demand is the assertion that if our senses were valid they would allow us to sense the real colors, temperatures, density, geometry, odors, sounds of objects ‘in themselves’ and not be limited to the subjective experiences that are the result of processing and filtering the ‘information’ sent to our senses by external objects.

Property Identity Fallacy versus the Proper Nature and Function of Perception:

This demand (a species of the psycho-physical property identity fallacy) is totally misguided, both biologically, and epistemologically. The facts are these: (1) The purpose of the senses is not to apprehend absolute physical properties of external reality, but to provide us with a set of processes for identifying only those properties of the external world, and our relationship to it, that allow us to effectively engage the world in order to explore it and live in it. (2) How this is accomplished involves a complex process depending on the simultaneous interactions between energy gradients, propagation mediums (air and water for example), sensory organs that register, process and filter the dynamic structure of simultaneous contrasting energy gradients across energy patterns and boundaries. (3) Necessary for this processing is an active conscious organism that is able to simultaneously engage with and respond to the object-specifying patterns in the energy flux registered by the organism’s sensory system, such interaction constitutes the necessary condition of sensory/perceptual ‘exploration and discovery of the environment.’ (4) As a consequence of the sensory/perceptual processing, what we perceive are not the simple energy mapped properties of the external world (i.e., their absolute energy values corresponding to physical wavelength, thermal intensity, acoustical frequencies, or baric pressure). Instead, our perceptual systems identify complex information features such as dynamic invariants within simultaneous contrasting energy values across sharp energy gradient ‘cliffs’. If this sounds like J. J. Gibson, it is because my views are deeply informed by that great perception scientist and by his brilliant wife and scientific collaborator E. J. Gibson. (See more about their work below. Also see my master’s thesis entitled The Causal Basis of Perception, based on Gibson’s theories of perception.)

To what do Perceptions Correspond?

As a consequence, the psychological experiences of such conscious events as ‘color’, ‘sound’, ‘touch’ correspond to complex relational physical constancies (contrasting rations of long to short wavelengths across a visual edge), rather than simple physical local constancies (wavelength, for example). Thus the perception of ‘red’ corresponds not to the detection of light waves in the 620–750 nm range (as many physicists and philosophers would have it), but rather the contrasting ratios of wavelength (or frequencies) across a physical ‘edge’ or ‘optical boundary’. That perception of ‘red’ is always reliable and always tells us exactly what long-short contrasting rations exist at that moment in that given optical direction in the real world. Thus, surprisingly, perception tells us far more than we expect it to tells us. It identifies a physically complex fact and not a physically simple one.

The Causal Basis of Perception as a Scientific Question:

While a valid philosophy can tell us that our perceptual awareness of the world is reliable and the necessary basis for conceptual ideation, it is the job of science, not of philosophy to study and understand the underlying causal nature of perception as a psycho-physical-biological developmental capability of the conscious organism. In other words, it is the job of science to discover how our perception of ourselves and the external world, specifically ourselves in relation to the external world, is achieved.

Gibson as the Key to Understanding the Causal Basis of Perception:

There is much science to learn before understanding how the senses give us true reliable perceptual knowledge of the external world. A good place to begin is by reading the works of J. J. Gibson, especially his brilliant and challenging thesis: The Senses Considered as Perceptual Systems as well as his magnum opus, The Ecological Approach to Visual Perception. J. J. Gibson is an advocate of epistemological realism (as contrasted with the many flavors of idealism on the one hand and materialist reductionism on the other).

In his 1967 paper New Reasons for Realism, Gibson wrote:

If invariants of the energy flux at the receptors of an organism exist, and if these invariants correspond to the permanent properties of the environment, and if they are the basis of the organism’s perception of the environment instead of the sensory data on which we have thought it based, then I think there is new support for realism in epistemology as well as for a new theory of perception in psychology. – Gibson - New Reasons for Realism, Synthese, 17:2 (1967:juni) p. 162.

In the Summary of this paper, Gibson wrote:

Both the psychology of perception and the philosophy of perception seem to show a new face when the process is considered at its own level, distinct from that of sensation. Unfamiliar conceptions in physics, anatomy, physiology, psychology and phenomenology are required to clarify the separation and make it plausible. But there have been so many dead ends in the effort to solve the theoretical problems of perception that radical proposals may now be acceptable. – op. cit., p. 171.

J. J. Gibson began working on the problems of visual perception with his future wife and collaborator Eleanore J. Gibson whose major extensions of J. J. Gibson’s work was her deep research into perceptual learning. E. J. Gibson extended J. J.’s work reflected in his The Senses Considered. Her first important book was Principles of Perceptual Learning and Development. Later her collaboration with J.J. on ecological visual perception inspired her magnum opus An Ecological Approach to Perceptual Learning and Development.

Sensory ‘Illusions’ as a Species of Functional Isolation and the Argument from Pathology

Returning to the issue of sensory illusions, the most important concept to realize is that no theory of perception can be built on theories of perceptual or sensory illusions. Why? For the very same reasons that no theory of biology or physiology can logically be built on the basis of biological functional pathology. Pathological systems are by their very nature malfunctioning systems. Pathology is explained as corruptions of healthy or properly integrated and functioning physical, biological or psychological systems. Illusions are the consequences of the carefully contrived ‘unnatural’, ‘deficient’, ‘constrained’ or ‘restrictive’ presentation of isolated visual (tactile, auditory, etc) sensory ‘stimuli’. It is the very fact that these presentation techniques prevent adequate exercise of the subject’s full perceptual capacities that leads to the ambiguity and vagueness that characterize so-called sensory illusions. When inadequate sensory information is presented to the subject, it should not be surprising that the resulting experience is to that very extent non-veridical and always associated with the feeling of ambiguity and uncertainty.

Similarly in the realm of neurophysiology, experimenters typically isolate some part of the nervous system (often using severe isolation) so that only the most primitive sensory-motor phenomenon can occur and be accurately observed and measured. It is a methodological illusion to believe that the results of such anatomical/physiological isolation, such as the extensive experimental work on ‘the reflex arc’, represent the primitive bases for and are revelatory of the neuromuscular principles underlying normal integrated behavior. This approach fails to recognize and appreciate the implications of The Integrative Action of the Nervous System (Sir Charles Sherrington, 1906) .

Similar errors occur in the study of embryological development, where local isolated phenomenon are mistakenly taken for fundamental biochemical elements of biogenesis. See the work of Paul A. Weiss, an innovator and mentor in the field of neurophysiology. Weiss once wrote: “Of what do we deprive a system when we dismember it and isolate its component parts, whether bodily or just in our minds? Plainly, of the interrelations that had existed among the parts while they were still united.” – from Paul Weiss, “One Plus One Not Equal Two” in The Neurosciences: A Study Program, pp. 801-821.

This fallacy of construction by structural or functional decomposition permeates much of science, particularly biological science leading to may false doctrines and theories. Underlying this general methodology is the dogma of mechanistic reductionism and functional or elemental reductionism, which holds that a system can be fully understood as the additive product of the actions of its individual parts.

Isolating sensory receptors thus preventing sensory exploration and proper sensory function can only lead to sensory-motor inadequacy and the common phenomenon of sensory ‘illusions’. We conclude that this method of sensory isolation commits the same methodological errors cited above, leading to fallacious conclusions regarding the functioning of our sensory-motor systems.

  • 1
    The body of this answer is a bulk copy of the blog post linked in the first sentence; this doesn't sit right with me.
    – Dave
    Nov 18, 2014 at 14:49
  • It's technically not against the rules as long as it's relevant (not spam) and not advertising, but it should be paraphrased. As it stands, it's far too much for a single answer.
    – stoicfury
    Nov 20, 2014 at 2:00
  • Dave - I agree that my posted answer was a bit long. But so was the question. It was not a pointed question such as 'Do Sensory Illusions Mean that We Cannot Trust our Perceptions?' Instead it was quite broad and very general. The questioner asks, at the end: "A good answer may also give me a good reference if this topic has already been discussed in depth." In other words, the questioner is asking for an in depth answer not a short pointed one. So perhaps my reply is appropriate to the questioner's request. Please reconsider your comment based on the above arguments. Best regards, Jack Nov 22, 2014 at 13:31
  • 1
    My comment was because I question (but don't condemn) the appropriateness of copying and pasting a blog post verbatim w/o indicating that that was done, not about the length per se.
    – Dave
    Dec 4, 2014 at 2:53
  • 1
    @JackSchwartz when I made my comment, I hadn't caught that you are the author of the linked blog post; making that connection in the answer is helpful.
    – Dave
    Dec 18, 2014 at 14:59

Illusion has nothing to do with intuition.

Intuition is the ability to acquire knowledge without inference or the use of reason. http://en.wikipedia.org/wiki/Intuition_%28psychology%29

That means that if I look for solution of a situation and it comes to me without any investigation or consideration of possibilities, and suddenly I know what is right, the knowledge came from intuition.

Usually it is believed that the knowledge came from subconscious processing of our sensory information and memories, or super-sensory information (/higher self) to which it is referred to in various spiritual communities.


Everything that comes in by our senses is just data, with no meaning. Then comes our conscious and subconscious interpretation that gives meaning to this incoming data. Thus, creating illusion in our experience.

That being said, we have no chance to perceive the external/objective reality, cause we are seeing only our interpretation of reality. Even science is proving that when you imagine a given thing, the same neurons are fired in your brain as when it comes through your sensory input.

There is a very good video on that on youtube named What the bleep

I think this is a very good question. It is clearly suggesting the need of knowing ourselves to be much more important than anything else. And it gets to be even more interesting when I start to realize who is perceiving my own perception.

  • There's a lot of contestable claims that you are making here. You're asserting one theory of consciousness and in the process seeming to miss something important as you wind through your answer. Intuition has more meanings than wikipedia is telling you -- especially in philosophy. See for instance how Kant uses the term: plato.stanford.edu/entries/kant-spacetime.
    – virmaior
    Dec 21, 2014 at 19:16
  • Jakub Mede. Your points on the subject of 'Illusion' contains so many contested premises that I would advise the reader to turn to the works of James J. Gibson and his wife and collaborator Eleanor. See especially 'The senses considered as Perceptual Systems' for many challenging answers to you premises. I have also written, in 1975, a lengthy theory of perception, that discusses the fallacy of 'Illusions' in depth. Republished by me here in digital form: bioperipatetic.com/the-causal-basis-of-perception Mar 26, 2015 at 14:43

Although this isn't a direct answer to your question, it's worth noting that several important philosophers have made optical illusions a central topic of inquiry. For Plato in the Republic, optical illusions are a metaphor for the deceptive nature of what we naively accept as reality. For Descartes in the Meditations, optical illusions are a spur towards seeking a foundation for knowledge outside the realm of sensory data.

In addition the well-known optical illusion called the duckrabbit is famously associated with Wittgenstein through his discussion of it in the Investigations, (although whether it truly counts as an optical illusion is unclear). I'm not expert enough on Wittgenstein to decisively state what he drew from the example, but you may find it worth your time to pursue.


If you are interested in the subject, I recommend the course "Introduction to Psychology" by Jeremy Wolfe, available on iTunes University.

Your eyes report signals to the brain. The brain has some very clever hardware to turn the signals from your eyes and turn them into useful images. And if you understand how this hardware works, it is possible to produce images that your brain will misinterpret. Wolfe actually describes one optical illusion that is so strong, it convinces your brain that it isn't working correctly and rewires itself; the effect can last for months.

Not really a matter of philosophy, but cognitive psychology.

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