# How does one tell apart left from right?

I can look at one hand and know (not necessarily immediately) that "This is my left hand".

How do I know this? (How can I be so successful at judging this?)

This question is inspired by the classic Kantian remark:

What can be more similar in every respect and in every part more alike to my hand and to my ear, than their images in a mirror? And yet I cannot put such a hand as is seen in the glass in the place of its archetype.

Left and right hands are almost identical but are not exactly the same. As Wittgenstein pointed out in 6.3111 of TLP, there is a transformation from one to the other.

But that doesn't answer the practical question: Is there a self-contained dictionary-like definition of right and left?

• How does one tell apart left from right?
• How do people come to know the difference? How do they learn it?
• Is this a mathematical question or an experiential one? Or some other kind of question?
• How would you explain the difference to someone who doesn't know about the distinction already?
• What original discussion of the problem has there been since Kant?
• These are two completely different questions. Abut your hand: You look at your hand, and you learn that the right one has the thumb one way, and the left one has it the other way. It's that simple, really. The other questions are much more complex and more mathematical than philosophical. Commented Jun 29, 2011 at 20:32
• How does one tell up from down?
– Dale
Commented Jun 30, 2011 at 7:58
• @JoeHobbit: for up/down, I feel there are simple to explain sensory devices that help distinguish them (feeling of weight, ease in movement). For left/right, I can't think of anything like that. Therefore the question. Commented Jun 30, 2011 at 12:32
• Re: the downvote, how can I make this question better? Commented Jun 30, 2011 at 16:50
• I was trying to get at the idea of arbitrary convention: Why did we not apply the word "up" to mean what "down" currently means - and vice versa?
– Dale
Commented Jun 30, 2011 at 16:53

A topological manifold is known as orientable when it has a concept of left- and right-handedness, so that a right-handed object remains right handed as it moves around inside the space. A non-orientable space, in contrast does not have this feature. One may think of higher-dimensional analogues of the Möbius strip

or the Klein bottle, which are non-orientable:

There may be little reason at first to believe that the actual physical universe is orientable, although we do experience this as a local phenomenon. But it is conceivable that a right-handed space traveler, having followed a certain path, would return as left-handed, while still insisting that he or she is right-handed. In such a universe, there would seem to be no fact of the matter about left and right.

Meanwhile, it turns out that some experiments show that some of the fundamental particles exhibit an asymmetry in their handedness, so that the right-handed forms interact with each other in a way that is different from what one would predict from the left-handed interactions. The wikipedia page on symmetry explains that this phenomenon arises only with the weak interaction.

With respect to these theories, then, one can tell left from right by carefully observing the weak interaction of various sub-atomic particles.

• This explains a microscopic, experiential method (helped by mathematics). Is there a more human-scaled method? Is there a supportable self-contained dictionary style definition of left and right? Commented Jun 29, 2011 at 17:50
• Given the comment about up/down, I think the mathematical description of surfaces goes a long way towards telling one hand apart from the other. But not specifically, say, right. I know i am writing with one hand, but how do I know I'm writing with my right hand? Commented Jun 30, 2011 at 12:37
• For a more human-scaled representation, use the Earth's rotation. The sun rises to your right when you face the south magnetic pole while standing near the equator. Commented Aug 9, 2011 at 20:21

There is a mathematical idea of handedness. Phenomena that are not identical to their mirror images are called chiral, and chirality has interesting roles in physics, chemistry, and biology.

It's not quite what you were looking for, but it's still pretty interesting.

• Michel Serres discusses chirality from a philosophical perspective in the beginning of Troubadour of Knowledge if I'm not mistaken. Commented Jun 30, 2011 at 2:45
• Yes, glad to see that someone has mentioned chirality. That's a very important concept here, especially with the classic example of left and right hands. If an object is chiral, it is non-superimposable upon its mirror image. That's how we distinguish our right hand from our left hand. And that merely stems from an application of our existing definitions of "right" and "left". There's nothing particularly tricky going on here. Commented Jul 1, 2011 at 8:04

Because the Right mouse button is on the right and the left mouse button is on the left...

To me Right and Left are learned. When I was young when i needed to distinguish right from left I remember picturing an Atari Console which had right and left players marked clearly. It is more rote now than it used to be but it still does not always feel natural. I sometimes get momentarily confused if someone gives me a direction like turn right if I am expecting to turn left. Other times I do not distingish the difference and turn left instead. If I have no expectation there are still times where I need to think about which direction that is. I actually know rmb and lmb not because of right and left but because it is in my muscle memory. I know left and right mouse buttons not because of the right and left directions but because that is how i have thought about them for nearly 20 years.

So basically left and right are nothing more than common labels. Much like drivers and passenger side. But I like to think of it as you are either right or wrong handed. And to be nice instead of saying wrong handed we said left... so going forward just replace left with wrong and it will be much easier.

• Yes, agreed. Classifying something as "right" and "left" is merely an application of existing definitions for those concepts. The real question is a decidedly more existential one: from where and why did those definitions arise as they are? Commented Jul 1, 2011 at 8:05
• Interestingly if you put right and left into google translate every language seems to have its own single word for them. translate.google.com/#en|it|right%20and%20left
Commented Jul 1, 2011 at 12:31
• Not to be argumentative (and it's an interesting point nevertheless), but most of the languages you're checking all share one or two common "root" languages. I see a lot of linguistic similarity between the respective words used across "families" of languages. It's likely that the concepts of right and left were formulated and became part of the language much earlier than any of our modern languages, and we're merely following in the footsteps of those who came before. Commented Jul 1, 2011 at 12:34
• I agree actually. It would be my logical assumption that the origins of left and right predate modern language.
Commented Jul 1, 2011 at 12:53

Thanks for all the great suggestions. I find the mathematical orientation answer the best but at some point that just says that you can show that two hands are of the same orientation or different, and still unable to say 'this one is the left one'.

I have convinced myself that, one can tell which is left and which is right by analogy with up and down. We can distinguish up and down sensorially by inner ear changes and proprioception (sense of contraction of particular muscles). It is easier to let the head drop down over the chest than it is to pick it back up; these sensations correspond to the sense of up and down (in space, one doesn't have that sensation, and one has difficulty assigning up and down that way.

As to left and right, one has visual sensors that are on one side and on the the other and those correspond (however one assigns them) to what one can call left or right (anatomically, when we look at the eye, we see that the part of the eye that corresponds to sensing what we call the left of the visual field happens to be on the right side of the eye, but that switch doesn't matter, we can still label what we see (not what we see with) by left or right consistently. (the same reasoning works with proprioceptive sensations in case of blindness).

This is all to say that one can identify left and right mechanically (operationally) by a mapping (simple or complex) to left and right sensors in the body/brain.

In some sense, the 'you just do it' answer comes closest ('the left hand is on the left') to being correct, but there's just no explanation in that simplification.

• I don't think you needed to make this community wiki. It's valuable to have a summary answer after a bunch of more detailed answers have been given. Commented Jul 15, 2011 at 20:30
• @Jon: I find it strange to expect to get the benefits of extra rep by answering ones own question. Community wiki allows voting on the answer without having the voter worry about my self-aggrandizing. Sort of a recusal from the process. Commented Jul 15, 2011 at 21:12
• "This is all to say that one can identify left and right mechanically (operationally) by a mapping (simple or complex) to left and right sensors in the body/brain." This would have been my answer. Gaze is directional, and object location (e.g. food or danger) is important to be able to not only identify but to communicate. Since humans aren't very good at waggle-dancing, we need to put location into words. Commented Aug 8, 2018 at 19:06

In JDH's answer, he discusses the phenomenon of orientability, in which a space 'has a concept of left handedness'- in truth it would be more acurate to say one can give the space a concept of left handedness- for there is always a coice of two.

This could be said to correspond, at least in two dimensional cases like the sphere (although it does extend naturally) to whether one chooses the inside or outside of the surface to walk around. Imagine two ants walking between two given points (close enough there is an obvious shortest route) on a soccer ball, one on the outside, the other trapped inside- at each point 'outside ant's' right is 'inside ant's' left and vice versa. (incidentally, this gives a nice way of thinking about non-orientable surfaces as surfaces having only one side)

Now you'll notice that, as you were visualising the inside and outside ants walking between two points, you were able (perhaps with some difficulty) to identify what they would call left and right. This is because for each ant I implicitly gave you a definition of what forwards was, and what up was- and you could have used the superficially satisfying definition:

Right is a quarter turn clockwise from forward, when viewed from above

Now this is only superficially satifying, as it rather leaves one wondering about how one defines 'clockwise'. In fact the two are tied up together- defining one will automatically give one a definition of the other, and sadly each is to all extents mathematically arbitrary (barring the aspect in which our mathematics emulates our physical world cf. the end of JDH's answer).

You see, the inside/outside correspondence I put forward toward the start of this answer secretly depends on the ants in question already having a left and right of their own that they bring to the table, so to speak.

Where this comes from, then, is perhaps therefore the body and brain of the ant (or human!), in which the somatic, sensory and motor functions of each side of the body are lateralized to the opposite side of the brain (that is, for example, the left visual hemisphere is represented in the right visual cortex), so the brain already knows its left from its right preverbally, and from there it's just a matter of association with sounds and shapes to 'learn' left and right as we do.

How do they learn it?

Whatever the answer is, it includes a lot of proprioception and body image. Via trial, error, and feedback, we learn that this kind of proprioceptive input is labeled "left", whereas that kind of proprioceptive input is called "right". We learn that this particular shape of hand is the "left" one, whereas that shape is called "right". As is mentioned elsewhere on this page, chirality is involved.

Proprioception, especially the part involving the head, must be strongly linked to what we end up calling the 6 fundamental directions - fore, aft, up, down, left, right. The stuff in front of our face is "forward"; the stuff facing this particular ear is "left"; and so on.

I'm not saying this is the only conceivable way that a conscious entity may develop these basic concepts, I'm saying this is likely how our species does it.

How do they learn it?

The same way we learn anything, by associating. Stuff happens on one side, we learn to turn that way. There's a path on one side, we need to communicate to others, so we name it. Clearly each side is different- different stuff is on different sides, so we name them uniquely. We name everything.

An interesting question is why there are 6 sides. Why not name 45-degree angles? Is there something fundamental about our orientation ability (brain wiring), or perhaps that as we get closer the angle changes so leftish is the best we can do? Or is it just practice, so we could learn to talk in horizontal clock angles or degrees?

• How does one tell apart left from right?

Hold up both hands with only your thumbs and index fingers extended. The hand that looks like an "L" is the left one. The other is the right one.

• How do people come to know the difference? How do they learn it?

Someone teaches them when they are children.

• Is this a mathematical question or an experiential one? Or some other kind of question?

Experiential. In order to understand the concepts of Left and Right, there must be some knowledge of sensory experience, because the concepts presupposes a physical world which we can only perceive by sensory experience.

• How would you explain the difference to someone who doesn't know about the distinction already?

"Hold up both hands with only your thumbs and index fingers extended. The hand that looks like an "L" is the left one. The other is the right one." If the someone was blind and/or didn't have hands, it would be necessary to present them with an analogous algorithm based on their sensory perceptions.

• What original discussion of the problem has there been since Kant?

They generally involve stoned people sitting in circles.

• How do you tell an "L" from a backwards "L"?
– JDH
Commented Jun 29, 2011 at 17:18
• @JDH, someone tells you and you remember. Commented Jun 29, 2011 at 18:37
• 'Just do it' might work but I'm interested in how that is possible. Commented Jun 30, 2011 at 16:49

There is no natural distinction between left & right, in the same way we can distinguish top from bottom, which we do because of the presence of gravity.

Your hands are arbitrarily named left & right. But once they're named they stay that way. And of course everyone else names their hands in the same way. So one can say the left & right distinction is socially constructed.

• Actually, this is only approximately true. One of the great discoveries of the second half of the 20th century is that parity is not actually conserved in weak interactions: nature does distinguish between right and left! There's also some suspicion that P-violations may be found in strong interactions as well, and searching for such violations is one of the lesser known experiments going on at the LHC. Commented Dec 8, 2013 at 23:29
• @DavidH: Sure, but phenomena on the atomic scale isn't, I think, the point of the question. Unless somehow that informs our sense of right from left, in the same some birds use quantum effects to have a kind of compass, apparently. Commented Dec 9, 2013 at 0:29
• Sure, the labels themselves are arbitrary like all words. And which one goes with which side is a social construction like all language. But it's not about what the labels are, it's about the concept. How do you know which one to label 'right'? We do it so it must be possible. Commented Dec 9, 2013 at 1:35
• @Mitch: Once you've labelled one side 'left' the other side, 180 degrees opposite is 'right'. I'm saying that it is convention that we decide what is left. We know it because we are taught it. In the microscopic domain, like David pointed out, there is an actual mechanism that objectively tells us that distinguishes left from right. Commented Dec 9, 2013 at 2:44
• Possibly interesting: I wondered how the Guugu Yimithirr speakers talk about hands. See here, here, but also here (p. 5): "[...] GY makes no use of locational expressions based on, for example, a right/left discrimination (although the lexicon distinguishes left from right hands, and left from right handedness)."
– user3164
Commented Dec 9, 2013 at 9:33

Suppose your left hand is induced some pain, why do you know the left hurts hand and not the right hand?

On the skin of left hand there are sensory neurons that senses pain. It is connected (half way through thalamus) to the cerebral cortex. Cerebral cortex is the area responsible for cognition. Now imagine there are some neurons that encodes the concept of "left hand". These "left hand neurons" activates whenever you are thinking about the concept of left hand. (search Grandmother cell)

Now it is easy, the left hand skin neurons is connected to "left hand neurons", and right hand to a different set of neurons, which in turn are connected to the English word "left" or "right" respectively, so that different words are screamed out.

• Yes, but the question asked for the situation that you see a hand.
– user2953
Commented Aug 12, 2016 at 4:24
• Suppose two dots is in the view field, one more left, one right, firstly they are projected to different areas of neurons on the retina, then ... apply the above argument. Commented Aug 12, 2016 at 6:38
• Then that is the argument you should make in your answer.
– user2953
Commented Aug 12, 2016 at 6:52

90% of human beings are right-handed. 10% of human beings are left-handed.

By the way, any bilateral animal has the concept of right as being different from left, they just don't need words for this.

Many species of snails curl for one side much more than to the other, in such huge imbalance (9x1) as ourselves.

Looks like this is explained by the Hardy-Weinberg genetic theory, but I'm not sure about the details.

For me the question has brought out the different senses of left and right, which may or may not be interrelated.

The distinction is between a question like:

How does a right-handed object relate its left-handed counterpart?

and one like:

How does a human subject know his right hand from left hand?

One way to bring out the distinction is to consider the statement in JDH’s answer:

There may be little reason at first to believe that the actual physical universe is orientable, although we do experience this as a local phenomenon. But it is conceivable that a right-handed space traveler, having followed a certain path, would return as left-handed, while still insisting that he or she is right-handed. In such a universe, there would seem to be no fact of the matter about left and right.

Suppose we accept that there would be no fact of the matter about left and right in such a universe. This alone would, however, not make the traveler’s insistence nonsensical. To him, his right hand remains the right one in spite of any orientation swapping. (Of course we don’t have to call it the ‘right’ hand.)

I believe the question of how a human subject knows the right hand from the left is assimilable to how he knows the right hand from the right foot, or the nose. Again, we can strip the questions of ‘right’ and ‘left’ and also of ‘hand’ and ‘foot.’ We can also strip them of epistemology by asking what it is for a sensation in the right hand to be in the right hand. Or if we are allowed the terminology ‘tactile location’ we are asking what individuates tactile locations.

We may ask the same question of ‘visual location.’ When I open my eye I am confronted with what looks like a surface. I can divide it into left, center and right regions, or upper, center and lower ones. I believe I could do this even if I were floating in space without any gravitational pull and didn’t have the benefit of my right hand often appearing in the right region or my hair often falling in from the upper. Of course I may not have the ‘right and left’ and ‘upper and lower’ vocabulary. Again we are speaking of individuation of visual locations.

I don’t think there is any necessity about the correlation between visual and tactile locations. In each of these, I think something fundamental is going on. That is to say, I don’t think one ‘learns’ that a pain is in one’s right foot and not elsewhere. ‘At first I only knew there was some pain, but didn’t know where. But by and by, I came to learn to locate them. I had to hire a teacher and practice it on my free time.’ This is not our experience; I don’t even know it makes sense to speak of ‘knowing’ or ‘not knowing’ there. In the same way, I don’t think we ‘learn’ that a thing is in the center of our visual field, or to the left, or to the right.

Of course, our visual and tactile locations happen to correlate. My right hand often appears to the right of my visual field. Or the sight of a dog biting the hand ‘over here’ in my visual field often (all right, always) correlates to my having pain ‘over here’ in my tactile field.

Suppose then I hold an apple in my left hand and see it in the corner of my left eye. The apple is ‘to my left in space’ as it were. This location in space would bear a certain relation to the ‘left’ region in my visual field as well as the ‘left’ region of my tactile field. We may say any number of things about this relation (not all of them right). E.g. the left region in space, existing independently, is sensed, tracked etc. by sensory events in me; is a ‘construct’ of my sensory locations; is mere shorthand for saying what happened in my sensory fields; etc.

According to this view, the concept of spatial left and right to a human subject would in some way or other derive from the locations in his sensory fields. The individuation of locations in a particular sensory field would be fundamental, meaning not the sort of thing one learns.

I believe that the visual experience may lead the way (as it were) in the assignment of location. That is to say, while my nose, neck, left and right arms and hands, the groins, the two legs and feet, the twenty digits, while all these are distinct without my having to learn the distinction, I don’t know that they stack neatly one way or other. For example, could I have known that my thumb is farthest away from the pinky without seeing the five fingers?

I don’t intend to tell a physiological or developmental story. I believe there would be a way to strip the thing above of all that stuff so that we are left with a thin analytical statement about the reducibility of spatial location to individuation of location in sensory fields, which is not a thing to be learned.

Such location would survive any orientation swapping as is imagined in the other answer because we are talking about two different things. I want to add that I am not sure about this, but only propose it as a possibility. I certainly have not argued for it. Thanks.