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I recall learning that humans have 5 senses, but that may have been an over-simplification to focus on the most notable ones, or simply that no others were known about at the time. Wikipedia says:

Although traditionally around five human senses were known (namely sight, smell, touch, taste, and hearing), it is now recognized that there are many more.

Is there a known, discrete number of senses (and, if so, how many are there)?

Note: If there's disagreement in the scientific community about the answer it would be interesting to learn what most biologists consider most plausible.

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    $\begingroup$ Downvotes? Really? Why!? $\endgroup$
    – stevec
    May 8 at 9:47
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    $\begingroup$ This post has 3 close votes as "opinion based" — probably be because this requires sharp distinctions among fuzzy groups. ——— In addition, the Biology.SE community has agreed that questions that show little or no prior research effort are off-topic on this site. Please edit your question and tell us where you've looked for answers, what you do know about the topic, and where exactly you still have questions. Under researched questions may also be subject to down-voting and closure. For example, you could start with the wikipedia on senses. $\endgroup$
    – tyersome
    May 9 at 5:06
  • $\begingroup$ @tyersome is it opinion based or just not well enough understood to have consensus yet? I presumed the latter. I did 3 things before asking: 1. Googled (lots of unreferenced articles I didn't trust), 2. Wikipedia'd it (see link above), and 3. Searched Biology SE (not previously asked here). I guess a 4th was look for an example of a nebulous biological concept (I searched for an article I read some time ago stating there were 13 different definitions of 'species', but I couldn't find the article so left that analogy out). BTW, if there's not consensus on this, that itself is an answer $\endgroup$
    – stevec
    May 9 at 5:48
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    $\begingroup$ This kind of question generally isn't interesting to professional biologists (in fact they are actively disliked). In part this is because even if you could answer it you wouldn't have learned anything particularly interesting (Let's say that it's 13 — so what?). This is also inherently dependent on how you define a sense and where you put the (somewhat) arbitrary boundaries among them — even within the traditional 5, smell and taste overlap. That makes it difficult (maybe impossible) to enumerate the "senses". More research could change this, but efforts aimed at counting won't get us there. $\endgroup$
    – tyersome
    May 9 at 19:05
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    $\begingroup$ @A.B. — Welcome to Biology.SE! It is unlikely that (m)any of the established users will see your question. The correct way to get that type of feedback is to post it as a question on the Biology Meta site with a link to this question (click where it says "Share" to get such a link). To me what you propose seems reasonable (depending on how brief and how well referenced your answer is), but I'm a relative newcomer and wouldn't want to lead you astray! $\endgroup$
    – tyersome
    May 14 at 17:55
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I'd like to challenge your question.

What is a sense?

Are you looking for a complex organ that integrates with the central nervous system? Or just any part of your body that can be used to determine the property of something in the outside world? For example, you can use your toenails to test whether a hot plate is of sufficient temperature to melt dead keratinous biomaterial, like your nails. This would count as an integrated sense for a specific physical chemistry.

Or do you think a sense is a thing that detects a singular property of the world? For example, do you discriminate proprioception from gravity sensing, even though it may be performed by the same organ? Historically, each classic sense is limited to one property, but this I think is outdated. For example, skin is not only capable of performing touch and texture detecting ("tactile" sense), but also heat, pain, and other things like sensing noxious poisons such as bee stings.

Also, do senses have to detect stimuli in the external world? Plenty of sensing goes on internally, both in interstitial spaces and fluids in your body, on the surface of cells, as well as within cells, and the variety of cellular and sub-cellular equipment is hugely various. Should light receptive cells only count as 'vision' or 'light receptive' when they are found in the eye, or can light-sensitive cells found deep within the body can also be classified as part of the visual sense? There are many animals and tissues in biology where light-sensitive receptors are not exclusively found in eyes.

These are only 3 things you did not consider. There are many other questions you must answer along these lines to have a clear, at least temporarily operational definition that can be used to make distinctions to answer what things can count as a separate, countable sense.

Just as a proof of concept, I can list more than five senses off the top of my head to show you that the "five senses" concept is woefully outdated in my view, and I'm sure some others would agree. I will pick some weird examples just to demonstrate the complications that the above questions illustrate.

Chemosensory systems

(considered a sense for chemistry? or a collection of senses?)

  • Olfaction (smell, as carried out by neurons in the nasal epithelium; e.g. smell of vanilla, and smell of bad food)
  • Gustation (taste, as carried out by neurons on the tongue; e.g. salt, sugar)
  • Antigen chemosensing (chemical sensing, as carried out by, for instance, immune antigen receptors on B cells)
  • Hormonal signaling chemosensing (chemical sensing of hormones such as insulin, as carried out for instance by myocytes)
  • Starch sensing? (amylase in saliva can be used as a test for digestable starch)

Visual system, at the retina?

  • Visible light (sensing electromagnetic radiation on the order of a few hundred nanometers in wavelength)
  • Internal methanol sensing (the visual system as a sensor for methanol, which disproportionately affects myelin surrounding the optic nerve)
  • Pressure sensing (see phosphenes)

The vestibular system

  • Gravity sensing
  • Balance
  • Coordination
  • Motion sensor
  • Head position sensor
  • Spatial orientation

Skin

  • thermosensation (touching a hot kettle!)
  • Nociception (pain sensing)
  • allergen sensing
  • sensor for gamma rays, X-rays and UV light (indicated by radiation burns, development of skin cancer, sunburns, etc.)

Bones and muscles?

  • Kinesthetic and bodily proprioception

Brain/mind/mental/social senses?

  • mental pain
  • boredom
  • mental or spiritual distress
  • sense of self and other, including friendship, power, place in social hierarchy, reputation, companionship
  • motivation and love (oxytocin, dopamine, etc. in limbic systems and other neural correlates)

I'm sure some would agree, and some would disagree about the specific cases I provide. Thus the definition of senses, or sensing, seems to be opinion-based or at the very least very sensitive to an agreed-upon operational definition, for which there is none.

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  • $\begingroup$ This is better and covers more things than what I was going to say (which was much the same thing, otherwise), so I won't. $\endgroup$
    – A. B.
    May 19 at 3:45

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