Eise

But this implies again that we do not see light... Really, the only way out is to acknowledge that the word 'seeing' has two meanings (at least). We both say 'I see an apple' and 'I see light', but the process as a whole is not the same. In the first case reflection is necessarily involved, in the second it isn't. But don't you agree that is just a question of semantics? Do you think we disagree on any physical process involved in seeing?

You said philosophy is not defined: So you are saying: We do not exactly agree on what philosophy is But science is a subfield of it. Well it is my opinion that, whatever philosophy is, it is not empirical science. I've studied both, and I can tell you, philosophy and empirical science are very different. Interesting enough your Wikipedia quote does not list 'empirical reality' as one of the matters with which philosophy is concerned. And before you come with cosmology as part of metaphysics, and metaphysics being a part of philosophy: today this is just not true anymore. Cosmologists are not necessarily metaphysicians anymore. Great, then you have learned something. Strange is completely right. A hypothesis is scientific, if it has empirical consequences, and is consistent with established scientific facts. A scientific hypothesis can be: Wrong, when the empirical consequences cannot be confirmed (Strange's examples: steady state theory, phlogiston, the aether) Correct (at least for the moment) when the empirical consequences are confirmed Open, when the empirical consequences are not tested (yet). It is true that scientific hypotheses demand empirical evidence. But that is because we want to know if the hypotheses are correct.

Ah, Really missed that. Just to be sure that I understand you: the Enterprise is flying in your direction, and without glasses you are seeing badly. So what do you see1: a blurry blue object. And that is because you see2 blue light. (I know what I am saying: I have glasses since primary school...). You are also consistently ignoring the fact that seeing macro objects is a different process as seeing light. It seems to me you are arguing that only see2 counts as seeing. But that doesn't fit to the daily use of the word 'seeing', in which we most of the time talk about seeing macro objects. I always meant macro objects when I wrote about 'things'. But if you want, I am flexible. So slightly different: We have the set of things. Now one can divide this set in subsets based on all kind of differences. One difference: there are things we see can because they reflect light, e.g. cars, houses and apples. On the other side there is one thing we can see because it enters the eye: light. The first category we can see1, the second category we can see2. (Didn't you say somewhere that 'we only see light'?) Then why do you have so much trouble that I clearly distinguish between the two meanings??? Of course. I repeatedly said it: no serious, real problem is solved by making this distinction. But one can make this distinction, and again, when somebody explicitly asks the question if light is visible, it should immediately ring a bell that this difference in use exists. So the correct way to answer such a question is to point to the two different uses of the word 'seeing', and that the answer depends on which meaning you give to the word seeing. If you imply both, then of course you see light and macro objects. And again you are contradicting yourself: on one side you argue that we only see light; on the other you say that both meanings are legitimate. So what is it? Then you must conclude that the daily use of 'seeing', in the sense of 'seeing houses, cars and apples' is the wrong meaning. Say the OP would have asked if it is true that we cannot see macro objects, would you have agreed with him, because we in fact only see light? I only described physical facts here. So what is wrong?

To come one time back to this: with the blue apple/light you are not describing something you really have experienced. You are describing a thought experiment. And with thought experiments everyone can look if he agrees with the ideas about the occurrences that are described. I am criticising your thought experiment, more or less based on the fact that you are begging the question. You get out what you put in. The relativity aspect does not add anything to the discussion. You just argue that the only correct meaning of 'seeing' is 'seeing2', seeing light. But that is already a presumption you put in yourself. And it does not fit our daily use of language where we also see cars, tables and houses. 'Cars are light?'. Now that sounds like faulty logic to me. Again: it is obvious that we all agree on the physical basis of seeing macro objects on the one side, and light at the other side. If we see light, it means the light enters the eye. If we see macro objects, the objects do not enter our eye. On the other side: we see cars because they reflect light, but we cannot see light that does not enter the eye, light does not reflect or emit light. Do we agree so far? Now my position is that this difference in meaning of 'seeing' does not create any problem: not in daily life, not in science, and not in philosophy. So it really is a futile discussion. But: when somebody explicitly asks if light is visible, and even shows his assumption behind the question, it is time to realise that there is a physical difference between seeing macro objects and light. And a possible answer would be 'Well, in the sense of ... of seeing, one could say light is invisible'. But immediately add that this is a bit artificial distinction, because we all know what somebody means when he says he is seeing light. Yep. That is also an interesting way of describing the difference. We can make this representation, because light enters our eyes from different directions of the macro object, and our brain constructs this representation from it, unconsiously (we do not have to think "oh, all this light, let's calculate if the light was reflected by some object. Aha, all that light is reflected by a single object! It's a car!"). You realise that use the word 'see' twice in this sentence. Do they mean exactly the same with the words 'see'? See my remark about thought experiments above. You see a blue apple, even if the apple in its rest frame is red. Light is not a macro object. I very clearly made this distinction. But for the sake of argument: what does 'being a subset' mean? It means that every attribute that applies to the superset, also applies to the subset. Now an attribute of macro objects is: can be seen by shining light on it. But this does not work for light. So light is not a subset of macro objects (even in this context).

Then you cannot see cars, because cars are not light. And it is still wrong. You see an apple: but for you it looks blue. So you see1 a blue apple. Of course this is because you see1 the apple because you see2 blue light. And you might guess well that this is due to the Doppler-effect, especially while Scotty's T-shirt looks blue to you too. But nothing in the blue light alone will give you a clue that the light is Doppler-shifted. Literally it is not faulty. As I said, there is objectively no preferred observer frame. Of course it is easiest always to take the restframe of the object under scrutiny and you, the observer, as preferred frame. But when there is no such frame, because the frame move against each other, who is right? Seeing things and seeing light are just different things. And when one reduces the meaning of 'seeing' to 'seeing things', then you cannot see light. And when you reduce seeing to 'seeing light' only, then one cannot see things that reflect light. And if you use both meanings for seeing in one word, then you do not account for the difference between how we see macro objects and light. Which nearly never is a problem...

I have this in mind all the time. Let's just look at the assumption behind the question 'Light: visible or invisible?' It is obvious: the OP's assumption is that we can see objects because they reflect light. If this would be the only meaning of seeing, then light really is invisible. You cannot see light that does not enter your eye, because light does not reflect light. (The same for emitting light: a light beam passing by does not emit light to your eyes. Therefore you cannot see light that does not enter your eyes). But I surely agree with you that it is a rather limited concept of seeing, because we are also used to say that we see light. We perfectly understand when somebody says he is seeing light. As you say, it has multiple meanings. But in the strict sense that the OP is using (seeing1), we cannot see light. In the normal daily sense, of course we can see light (see2). And mostly we do not bother about this distinction, because it seldom leads to confusion. Except you meet a philosopher in a thread about the visibility of light...

One could do that. Your 'sensing' however would be the same as my seeing2. But if you do not introduce a different concept for 'seeing1' the confusion will continue. One should exclude the meaning of 'sensing' from 'seeing'. To clear this up I am using 'seeing1'. Your use of the concept 'semantics roadblock' is interesting. If one wants to try confusions as illustrated in this thread, one should be prepared to introduce new concepts for words, making them more precise, and sometimes introduce new ones. But obviously the resistance against such semantic moves is very strong. A semantic roadblock, as you say. Yep. Therefore I introduced non-existing words: see1 and see2.

Wrong. You see1 a blue apple. Only observers in the same inertial frame see1 the apple red. There is no objective reason to say that some inertial frame is preferred above another. So once again: do you see3 the difference I make between seeing1 and seeing2? And do you see3 that there is a difference between seeing light and seeing objects by means of light?

Oops, that would be see3... You don't see1 yellow light. You see1 a bright yellow tube. I think that if you want to have a clear discussion, you must distinguish between seeing1 and seeing2 (forget about seeing3...). As long as you mixup the two, we cannot have a clear discussion. I see1 the yellow tube, because I see2 its yellow light. That it accounts for the difference how we see macro objects (that do not need to enter the eye to see them: we see them by the light they reflect or emit), and how we see light (because it enters the eye). Do you see this difference, and do you see how I use the words 'seeing1' and 'seeing2' to account for this difference? Exactly. But if you want to distinguish between these 2 ways of seeing, then one make such a (artificial, I fully agree) distinction. As it solves no real physical or philosophical problem, it is of course an empty intellectual exercise. Yes. Kirk sees both still red. But you see1 Scotty's shirt and the apple both blue. Really, tell me what you see. And I am sure: you see a blue shirt and a blue apple. But you see them thanks to two different processes. But both fall under the definition of see1. When the lamp is turned on, you just see2 no reflected light anymore, you see2 yellow light. So you see1 a bright yellow lamp. Don't you see that seeing light and seeing macro objects are not the same? Yep. That's the whole point. But that is wrong again, and inconsistent with the sentence I quoted from you above. 'Seeing' in daily life includes both meanings. And then of course it is at least very funny when somebody reduces the meaning to seeing1, and therefore concludes that light is invisible. And then furiously arguing that seeing1 is the only correct meaning of seeing. I hoped that distinguishing between both meanings would clarify the discussion, but obviously that did not work... So once again: do you see3 the difference I make between seeing1 and seeing2? And do you see3 that in daily life we do not make this difference, and so we can see things, but also light.

I do not see a problem. Seeing1 is by means of reflecting light by an object, or by emitting light by an object. Sodium vapour is blue when it reflects light, and it is yellow when it emits light. In both cases you see1 a glass tube. Of course, you must see2 ('detect') the light to see1 the street lamp. But this is of course all very artificial...

Ok, this is what John said: What you see1 are two different things: - a street lamp with a blueish content, by means of reflecting light - a street lamp with a bright yellow content, by means of emitting light But of course you see2 blue light in the first case and yellow light in the second case.

What did I miss? If you imagine the observer, then what he will see1, is that the apple is blue. You see that because your visual system sees2 blue light, instead of red. Just shift the meaning of to see to see1 and I addressed John's argument. Furyan5 used the words 'to see' and to 'to detect'. But that leaves his absurd sounding 'one cannot see light' standing. With my artificial difference between 'to see1' and 'to see2' it is always clear what I mean: You can see1 macro objects, thanks to reflecting or emitting light. We cannot see2 macro objects. We can see2 light, but we cannot see2 macro objects because they do not enter our eyes. Everything clear?

It is not defined exactly. If it really would not be defined, your OP would be meaningless: science is a subfield of 'something undefined'. How can one decide such a question if we do not agree about what philosophy is at all? It once was. Modern metaphysics has nothing to do with that. See e.g. David Armstrong.

That is simple: you see the colour of the apple change. Not of the light itself. It all boils down to the difference between how we 'see' different things: We 'see' the macro objects around us, because light is reflected by them or they emit light themselves We cannot 'see' light because it reflects light or it emits light; we can 'see' light directly, if it hits the eye So in the first point light is just the means to see the things around us. But using light to see light simply does not work. (What do you do in a hell lit room, with a very faint light source? Do you turn one more lights, or do you turn the room lights off? What the hell is that, something you see better in the dark?) Furyan5 furiously pleads that the second meaning of 'seeing' does not count as seeing. If it would be an important philosophical topic (it is definitely not a physics topic!), philosophers would split up the two concepts of seeing with the help of two different words: one can 'see1' macro objects, and one can 'see2' light. And then try to solve the real problem behind it. In this case however, I do not see any real problem. We all agree more or less on the facts of seeing. But it is a nice intellectual exercise: try to understand the opponent, find different concepts that can lurk behind the same word, and so try to light up the intellectual problem, see the root cause of the seemingly different views on the facts. In this case it really is nothing more than an intellectual exercise. ('Light' metaphors used intentionally...)

Worthwhile? At most in the context of this discussion... To your question: Yes, there is. If you see a car, then you see it because it reflects light. In such a case it is pretty unusual to say "I see light...wait... from the light I deduce that it is reflected by a car. So there must be a car there!" No, you would say "I see a car". Now try to see a light beam passing next to you (so it is not detected by your eyes! No single photon of the light beam enters your eyes, it is not shining on a screen or a wall). Shining light on it will not help you to see the light beam. In this sense (but only in this sense!) one could say that light is invisible. You cannot make it visible by pointing a torch at it, as you could do with a car. Worthwhile? Facts are expressed by words. If we can agree on the meaning of words, only then we can possibly agree on the facts. Otherwise the same sentences mean different things to different people. Maybe it would help to formulate the facts unambiguously. So let's try 2 contrasting facts: You can make a material, macroscopic object visible by shining light on it (e.g. a car). You cannot make a light beam (that passes by) visible by shining light on it. Do we all agree on these 2 facts?

I am wondering if people here disagree on some physical or biological facts, or just the semantics of the the verb 'to see'. If the latter, is such a heavy discussion necessary when all agree about the facts?

That still means that every opinion in philosophy is equally valid. Do you really think that? Example: 1. Humans exist of 2 substances that can exist independently: res extensa and res cogitans, which are interconnected in the pineal gland. (Cartesian dualism) 2. The mind is a function of the living brain. You think these are equally valid?

Sorry, had to express me a bit clearer. If we say 'I see a light', we mostly mean that we see something emitting light: lights of a car, or a torch, or whatever. But there are some other cases, e.g. if you see light at the end of a tunnel. But generally you are right, we see things because they reflect light. The colour of the light bulb? Of course a torch doesn't light itself.

I think you nailed it, MigL. When it is dark in a room, I can see things by turning on the light: they reflect photons. But I cannot do the same with photons themselves. If there is a light beam in the further dark room, I cannot see the light beam. I still cannot see it if I turn on the light (it becomes even worse...) If we 'see' light, we mostly mean we see something that emits light. If I look into a beam of a torch I do not see light: I see the torch. But if we do not recognise the source of the light, it is very usual to say 'I see some light'. But I fully agree that it is semantics. It is clear that if photons are detected by our eyes, we see 'something'. So it is very understandable that we say we can see light. In my opinion however, the question is not of much importance. So I have no idea why the emotions are cooking so high. I do not see how deciding for the correct semantics any important scientific or human problem is cleared of solved.

So in philosophy, one learns a lot of opinions, and there is no argumentation about the validity of these opinions. Every philosophical 'opinion' is just as valid as any other? And that is the reason one can study it at university level? That it is wrong. The topic of science is empirical reality: physics studies matter, chemistry studies chemical reactions, biology the living nature etc etc. Philosophy studies our way of thinking, tries to find out which ways of thinking lead to valid conclusions. Somebody who studies matter is a physicist; somebody who studies how a physicist comes to valid conclusions (i.e. reflects about the scientific method), is a philosopher. (Of course somebody can be both). Another example is morality: somebody who asks what action seems moral to him is considering morality. Somebody who reflects on how people argue for how we think morally, and what kind of arguments may lead to valid moral propositions is an ethicist, i.e. he is doing philosophy. It seems to me that you see scientific speculation as philosophy. It is not. Good scientific speculations lead to predictions that can be empirically tested. And even scientific speculations where we have yet no idea how we can test them, are still not philosophy, because of their topic: things in reality, not our way of thinking.

Sorry, I am just looking at the text. Einstein very clearly says: And these are the principles: From these principles he derives the rest, including the independence of the velocity of light for the observer. I assume he could have done it the other way round: take observer-independence, and from there derive emitter-independence. But he clearly does it the other way round. No idea why you think that. When from both postulates follows that for every emitter and observer the velocity of light is c, then it is clear that in this article special relativity 'is born'.

Every science has its own domain and methods. E.g. functional explanations have nothing to do in physics, but they are very important in biology. Philosophy has its own domain, namely our way of thinking, so not empirical reality. For empirical reality we have the sciences. However, as soon as science reflects on its own concepts and methods, it is doing philosophy. As I said in some other thread, the topic of physics is not physics, but certain aspects of empirical reality. I did not say that, that was Phi for All. And I also do not agree. The 'why-question' is much too ambiguous: why does the stone fall? Because of gravity. That is definitely not philosophy. If we ask for reasons, not for causes, then it still can be exact science: why do mammals have a heart? To let the blood flow through their bodies. Still not philosophy. No. Why you laugh is a question of psychology, which is also a science (at least it tries to be). If it is justified to laugh might be a philosophical question (an ethical one, maybe. Some jokes are terribly tasteless, on the cost of Jews, Blacks, Muslims, women... you name it.)

Well, I think the 2. postulate is clear: it is about the emitter of light. The location you cite seems the derivation to me, that it also means that the velocity of light is the same for all observers. I am over-asked. (Is that English?). I assume Einstein wanted to start with postulates as close to the normal ideas of physics in his time. Independence from the emitter must not have been a problem, as waves propagate with a fixed velocity in it medium. But how he gets at independence of the observer is not clear to me.

Well, the definition of a description is different from a definition of a definition. If you know what I mean...

Well, I think it is best to let Einstein speak for himself: So in the second postulate he really is referring to the emitter only. I assume this excludes two (logical) possible scenarios: Light is moving in a medium, like sound. A jet close to the velocity of sound will 'see' the sound moving slower from him in the direction of his flight. If light exists of particles (material) the velocity of the emitter should be added. I found it always interesting that in popular explanations usually the observer is considered, not the emitter. Wonder why. Because it is easier to explain?