Descartes says that the senses deceive us. I agree for all senses, except for the tactile sense (i.e., when you touch something).

It's clear there are optical illusions, but there appear to be no tactile illusions. When we touch something, we know or we don't know (and know that we don't know) information about that thing. I can't remember being mistaken by my sense of touch.

Can anybody give an example of when the tactile sense deceives us?

  • 3
    Descartes' radical doubt considered the possibility of our senses deceiving us, then concluded that because God exists and God is good and perfect, he does not deceive us, since deception is a sign of imperfection. – Ben Jun 1 '13 at 21:39
  • @ChaosAndOrder I think Descartes talks about God because, otherwise, he would be burned by the church. Anyway, I'm still only analyzing the previous thought "senses deceive us" and it's legitimacy. – Tom Brito Jun 1 '13 at 21:44
  • 1
    This sense is perhaps the most illusory in that objects which feel solid to the touch are in fact mostly empty space. What you're actually feeling is the electrical repulsion between the surfaces. – David H Jun 1 '13 at 22:00
  • 1
    @TomBrito I personally don't think Descartes talks only about God simply because he'd be burned by the church otherwise. He could have 'simply' not mentioned him instead. – Ben Jun 1 '13 at 22:09
  • @DavidH it's a good point! My thoughts on it: with only the tact, we really do not talk about an object being solid (like a cube, which we would knock or press to try find out if it's solid). Now talking about surfaces, if we understand for solid something that have a given level of electrical repulsion (maybe more than our skin would be the boundary?), do not flow, and do not expand, then looks like the tact is correct. Right? – Tom Brito Jun 2 '13 at 0:11

Touch is just another form of sensory input subject to imperfect reading of the world like any other sense. http://en.wikipedia.org/wiki/Tactile_illusion

The whole phantom limb phenomenon involves massive deception, not sure whether this fits in your categorization of "tact". (Phantoms in the Brain: Probing the Mysteries of the Human Mind - fascinating and unexpected)

It's interesting that the sense of touch is a main element in a parable about the fallibility of senses and knowledge: https://en.wikipedia.org/wiki/Blind_men_and_an_elephant

| improve this answer | |
  • Thanks! These a really interesting links! Looks like "tact" is a too embracing term. In the future I'll try to split it into small parts to analyse. But before that, I'll try to read carefully each of the examples, and remove all the examples where the tact actually do not give you sure (like in the blind men and an elephant), so you couldn't say it's lying to you, right? (e.g. tact will never lie about the texture, but may lie about the "what's it" information - which is actually a deduction) – Tom Brito Jun 2 '13 at 19:21
  • @TomBrito - how useful is "lie" in this context? The fidelity of the information varies. By touch I can differentiate between an 80 grit sharpening stone and 200 grit, but I can't tell the difference between 8000 and 10000 grit. My sense of touch is lying to me when it tells me the texture of 8000 grit has the same texture as 10000 grit, and will fail me in proper sharpening of knives. I bet blind people could tell many stories about the erroneous nature of touch. – obelia Jun 2 '13 at 19:42
  • Ok, my use of "lie" was not much clear. So, let's use Descartes first law (on part 2 of his Discourse on the Method), "never to accept anything for true which I did not clearly know to be such". So, we can rely on our tact in some level, not in all the levels, agree? And we can't rely on tact to say "this is" an elephant/wall/etc, as your example say. But when with only your tact you can really say to yourself "I'm sure", it's correct, agree this far? – Tom Brito Jun 3 '13 at 11:47

Temperature may be a good example. If you give someone a metal bar and a book, which both have the same temperature, say 15°C, people will say the metal bar feels colder than the book. It's different from an optical illusion in the sense that there is a real, physical explanation, namely that (a) the temperature we feel is the temperature of our hand, not of the object we're holding and (b) the metal bar is a thermal conductor (it will drop the temperature of our hand faster), while the book is a thermal insulator (it will drop the temperature of our hand only very slowly).

Videos that illustrate the point I made in a very clear manner:

| improve this answer | |
  • The second video link is broken. – Tom Brito Jun 1 '13 at 23:01
  • @TomBrito Not anymore ;) – Ben Jun 1 '13 at 23:04
  • Its a great point to think about! The first video explains that "hot" and "high temperature" are different concepts. Thinking about, looks like its a mistaken interpretation if you say "the temperature is high", while your tact is just saying "it is hot", meaning it is giving out a lot of energy. (sharing my thought, but I'm still thinking about) – Tom Brito Jun 1 '13 at 23:09
  • @TomBrito There is of course a relationship between high temperature and hot feeling. A high temperature is necessary but not sufficient to feel hot (i.e. if it's a book that has a high temperature, your hand won't feel hot, if it's a metal bar that's hot, your hand will feel hot). – Ben Jun 1 '13 at 23:18
  • "people say the metal bar is colder than the book", and they're right, as cold means "absorbs a lot of energy". But, if someone say "the temperature of the metal is lower then the book", it is wrong. And looks like it's not a mistake of the tact, the tact (for temperature) says whenever it's hot or cold (indirect, by the energy on our skin). Looks more like a mistake of our interpretation of the information. In opposite, in a optical illusion, the information itself, given by the eye, is wrong (not the interpretation). (by the way, please, even if you agree with me, do not delete the answer) – Tom Brito Jun 1 '13 at 23:39

Yes, cross your two middle fingers. Then close your eyes and rub your nose between the V made by your two crossed fingers. You will have a sense of having two noses.

| improve this answer | |

Way back when I was a child, I watched Mr. Wizard on TV. He had a neat experiment to demonstrate how touch can deceive. He took a ruler, and made a cardboard slide that goes around it. At one end of the ruler, he put a pin, and he attached one to the slide, so that he could vary the distance between the pins.

He got his subject to hold out her arm and he lightly tapped the pins against the inside of her arm, an inch apart at first, and asked her how many pins she felt: two, of course. He then decreased the distance until he got within the distance we have between sensory nerves in the skin. At that distance, suddenly one pin vanished, and the woman helping him only felt one pin. (he then went on to repeat the test on other parts of the body, such as the lips, to show that different parts of the body have sensory nerves closer together than others).

Also consider the fun experiment where you stand in a doorway and press your arms out against the sides. Spend a minute like this, during which your body is processing its tactile inputs and making its best guesstimates as to what your body is doing. When you step out, your arms rise. You end up confusing the sensors in the body which detect muscle length, which are a fundamental part of the kinesthetics that tactile input lets us use.

Many internal martial arts take advantage of touch's ability to deceive, allowing their opponent to overextend because their opponent thinks they're in one position when they're actually in another.

Finally, saving the best for last, why not just take the lesson from the best: Penn and Teller.

| improve this answer | |

In opposite, in a optical illusion, the information itself, given by the eye, is wrong (not the interpretation).

that's incorrect, imho. eyes give you information as they receive it. it's your mind interpretation that's wrong. that's true for any sense - you get the raw information, and then you mind misinterpret it, based on your previous experience

just one interesting example from the psychology - afaik, newborn cannot distinguish separate objects in that he see, he even can't focus on them! it needs some experience to start perfrom these activities, and that means that unless you don't focus and don't try to recognize separate things, you are interpreting information your eyes can give you

| improve this answer | |
  • i could add that any sense can't deceive you - it just sends through the information it gets. for example, vision pass you a matrix of color points. it's up to the mind to interpret this information and up to the mind to consider its own interpretation errors as belonging to the sense or the mind itself. So i agree with Descartes - since sensual information without interpretation is absolutely useless for us, senses will always deceive us. just because we think in the terms that exist only in the mind itself – Bulat Jun 24 '13 at 19:08

Yes, similar to 'The Doctor's' answer above but more to the "point": The following is called "Aristotle's Illusion." This "tactile illusion" occurs when the eyes are closed, and the index finger is crossed under the finger next to it. Then the point of a pen or a pencil is placed directly into the cleft between the tips of the crossed fingers. The sensation is that of touching two objects rather than one. Try it. You can do this with or without another person. You will find, since your eyes are closed, another person can more easily place the point directly into the cleft... If you would like an explication / explanation you can find it on the "Merleau-Ponty and Phenomenology' fb group page. Seeing and touching "overhang" each other more than any other senses. All or most of what we see is already from afar inscribed in our tactile field. Likewise all we touch reaches over into vision insofar as sleek and rough surfaces are available to the visual field even while tacit. (As indeed the second layer of meaning - "vision" - indicates.) Finally, (this is no joke) ALL vision is 'TELE-vision,' literally!

| improve this answer | |
  • There's a lot of random stuff in this answer. Can you edit it to be just about answering the question at hand? – virmaior Dec 27 '15 at 1:49

Not the answer you're looking for? Browse other questions tagged or ask your own question.