Merrill Ring

Suppose a philosopher, trying to show both that and how scientific discovery alters our concepts, turns to the topic of light as follows.

'It is hard to deny that there has been conceptual change and that such changes result from scientific discovery. Consider the concept of light. Once one would have explained what light is by saying "Light is that which is makes things visible". We would no longer say that in explanation of light: for we now know that not all light is visible. Visibility has dropped out of the concept of light after the creation and acceptance of electromagnetic theory. A thinker in, say, the seventeenth century will not permit the expression "invisible light" but we do. Both are concepts of light, but clearly different concepts.'[1]

Did electromagnetic theory (EMT) amount to a change in the concept of light as held in the above argument?

The present inquiry cannot be a fully satisfactory answer to that question. For to be such would require that we (more or less) share a (more or less) defended account of what a concept is - and that shared background certainly is not presently available and would hugely distort the present paper were such an account to be introduced and defended here.[2]

Even more specifically, to answer the question decently would require a defended answer to the philosophical question 'What is light?'. That is, given some agreement as to what a concept is, it would be necessary to fully answering that question to set out what is involved in the concept of light. Such a philosophical investigation in all its details seems never to have been undertaken. Any attempt to do so also falls outside the scope of this paper.

Despite those major shortcomings, I shall persist in addressing the question of whether the creation of EMT involved a change in the concept of light. This inquiry may produce not only a better grasp of that issue, but also of the larger background topics necessarily omitted here. At the very least, we may come to appreciate some of the issues that separate those who think that EMT unquestionably altered the concept of light and those, such as myself, who are inclined to question such a thesis.

It should be noted at the outset that the argument set out above is a kinder, gentler version of a position expressed by Paul Churchland. What I shall do is to introduce the Churchland material later as a contrast to my initial foray into the topic of light and EMT. Until then, treat the only relevant discussion as that sketched at the beginning of the paper.

Now that argument hinges upon the phrases 'invisible light' and its contrast 'visible light'. To put the argument succinctly: the phrases 'visible light' and 'invisible light' have come to make sense and to mark out natural phenomena because and only because of the discoveries embodied in EMT; hence the scientific creation of EMT has altered the concept of light.

Let me start the inquiry with some details of EMT, construed in the most straightforward way. At least I so designate them – Churchland will think the presentation misguided. Then we can see what should be made of them.

EMT is the discovery that electromagnetic radiation, emitted by different objects in the universe, exists in a spectrum with wavelengths running from just above 0 nanometers (nm) to waves with a length above 1 meter. Light, which affects the optic systems of humans and other earthly creatures and enables us to see, forms one segment of that spectrum. Other types of electromagnetic radiation, i.e. radiation with other wavelengths and consequently other properties, occupy other portions of the spectrum. It is those categories of radiation and their names which are of importance to us here.

It is not clear how deeply EMT is committed to the current specifications of the categories and to their current names. As one textbook says: "To make discussing [the range of electromagnetic waves] manageable, scientists have divided the full electromagnetic spectrum (or range) of such waves into some arbitrary categories."[3] That is, there is some reason for holding that the point of the categories is the manageability of discussion. The implication is that the categories employed are not to be thought of as natural kinds. Further, nothing is said in that text about the names affixed to the categories.

That portion of the electromagnetic spectrum falling between approximately 400 to 700 nm is called 'light' in the categories employed by science. That range of wavelengths is both marked out and so named (obviously) because it is the range of electromagnetic waves "which human vision is sensitive to."[4] Notice that the EMT category 'light,' as so explained, is quite homo-centric, and that in two ways. It first ignores the fact that other creatures on earth have evolved sensory structures sensitive to that same range of radiation. However, more importantly and more centrally to the theory, that portion of the spectrum is marked out for special notice because of its importance to humans. Hence this category and its title are not introduced solely for the manageability of discussion any more than the category and its title represents a place where nature divides at the joints. The category light and its name are thus reflections of human capacities, practices and interests.

EMT then proceeds to set out the further categories of electromagnetic radiation that fall on either side of light. On the one side, light radiation is bounded by ultra-violet radiation, followed in order of decreasing wavelength by x-ray and then, with the shortest wave lengths of all, gamma ray radiation. As the electromagnetic waves grow longer than 700nm, there is found infrared radiation, then microwaves, VHF-UHF, shortwave radio and long-wave radio radiation.

So much is the straightforward presentation of EMT that I promised. It is important at this point to introduce Churchland's account of the theory.[5] It must be noticed that Churchland does not argue against my 'straightforward' account of EMT nor even mention it. His view is presented as the only one.

The central, and practically sole constituent of Churchland's presentation of EMT, is that light is electro-magnetic radiation. That claim is construed, as is to be expected on the basis of other views of his, as an identity statement. "...light reduces cleanly to EM waves."[6] "...light is simply identical with EM waves."[7]

Set aside the issue of whether there are identity statements in the neighborhood.[8] The crucial point for present interests is that in my account I have held that what has been scientifically discovered is that light is EMR of wave-lengths between 400 and 700 nm – with the consequence that most EMR is not light. Churchland, on the other hand, holds that, according to EMT, light is to be identified with the entire electromagnetic spectrum, from near 0nm to above 1 meter.

To be brief (and with injustices to be corrected soon), Churchland is simply mistaken about this. Investigation of the physical constitution of the universe has discovered that light is a kind of electromagnetic radiation, differing from other forms in its wavelengths and its possibility of affecting the optical systems of various creatures on earth. However, it has not been a scientific discovery that light includes portions of the EMR spectrum other than that narrow band between approximately 400 to 700 nm. No one has found out that ultraviolet, infrared, gamma rays, radio waves, etc. are really forms of light. It has not been learned by physical investigation that some light, even most light, is invisible. Physicists and astronomers have not, from their sophisticated inquiries, come to know that 'light' covers the entire range of EMR.

Hence Churchland is mistaken when he says that anyone who speaks of light as necessarily visible is "...someone still imprisoned by pre-scientific prototypes of light. Invisible light may well be a conceptual impossibility against the assumptions of the story just told, but we now know better. Indeed, we have learned that most light is invisible – and not just 'shallowly' invisible, but permanently beyond human visual apprehension.... Once again, we find ignorance being paraded as positive knowledge."[9]

So far what has been accomplished is that I have said that such and such is way things are in consequence of EMT and Churchland has said that that is not the way things are and that to think as I do is to be "imprisoned by [a] pre-scientific" view. The first thing which needs to be done is to see why Churchland denies my claim that light is EMR of between 400 and 700 nm and holds that it is identical with the entire EMR spectrum. To discover that requires looking into some scientific texts on the topic of EMR – for it is from them that Churchland has acquired his view.

I will rely upon three astronomical texts in which the contemporary understanding of EMT is set out. As we are aware today, post-Kuhn, if you want to see how a matter in science is understood by the disciplinary community, look to standard texts. There are many such texts, but I have no reason to suppose that there is any significant deviation from what I have found in the three I have selected semi-randomly.

Let me consider first the discussion of EMT in an orthodox text, Voyages Through the Universe (see note 3), used by my son in a standard college astronomy class.

In the chapter 'Radiation and Spectra', the authors begin talking about "light and other forms of radiation" on the opening page (p. 81), proceed through a discussion of Clerk Maxwell and the creation of EMT, the wave theory of light, the Hertzian work that led to the particle theory of light – a discussion which includes the sentence "Reluctantly, physicists had to accept that sometimes light (and all other electromagnetic radiation) behaves ...."[10] In that sentence, light is contrasted with other categories of EM radiation.

Throughout the opening discussion, then, the text contrasts light with (all) the other forms of electromagnetic radiation. Light is (only) that portion of the EM spectrum having such and such wave-lengths, the description which I have employed in my version of the theory.

However, in the next section, entitled 'The Electromagnetic Spectrum' which examines in more detail the various types of EMR, there is a change. In the detailed discussion of ultraviolet radiation, some new terminology regarding light is introduced although without any explanation, especially without any notice taken of the conflict between the ways of talking now employed and how the discussion of EMT began. We find there the sentence "Radiation intermediate between x rays and visible light is ultraviolet".[11] 'Light' there acquires an adjective, 'visible', and thus there is created the possibility of there being some kind of light other than what we require in order to see. The detailed discussion of what had, in the introductory material, to the chapter been called 'light' simpliciter is now conducted in terms of 'visible light': "Electromagnetic radiation with wavelengths between roughly 400 and 700 nm is called visible light...."[12]

With the adjective 'visible' admitted in qualification of 'light', can 'invisible' be far behind? In this particular text, that adjective isn't emphasized, though it is present. When discussing ultraviolet, the authors say "Outside the world of science, ultraviolet light is sometimes called "black light" because our eyes cannot see it."[13] That is, what had begun as a form of EMR, ultraviolet, to be contrasted with light, has now, without explanation, become a type of light although we cannot perceive ultraviolet with our eyes. It is but a short step from there to the explicit use of the word 'invisible', although in this particular text it is never used as an adjective qualifying 'light'. The adjective does occur, however, in the discussion of infrared radiation: "This was our first hint about the other (invisible) bands of the electromagnetic spectrum."[14]

The second science text I am employing here is different from Voyages in that it is more extreme in its expansion of the category of light. The book is entitled Astronomy Notes.[15]

The title of Chapter 7 there is 'Electromagnetic Radiation (Light)'. Now that might be taken in either of two ways. It might introduce a chapter on EMR with a parenthetical note that light will be discussed here; or that might be said as a way of noting that the two words will mean the same in the ensuing discussion. That latter reading will turn out to be the correct one for understanding this particular text.

The introductory blurb to the chapter says "At least 95% of the celestial information we receive is in the form of light. Because of this fact, astronomers have devised many techniques to decode as much as possible the messages that are encoded in the often extremely faint rays of light. .... Roughly 85% of the information in light is uncovered by using spectroscopy---spreading the light out into its different constituent colors or wavelengths and analyzing the spectrum. The first part of this chapter covers the characteristics of all forms of light." It first appears that it is light, taken straight up, which provides the overwhelming majority of celestial information. For it is light, so taken, which can be separated into constituent colors. However there is some ambiguity, especially given the chapter title and now the final words: how are we to take the phrase "all forms of light"? There has been no prior intimation that light has forms unless it is the wavelengths of the different colors.

Once again, after that seemingly straight-forward, though possibly ambiguous beginning, as soon as the discussion turns to other forms of electromagnetic radiation, new terminology slips in which compromises the idea that light is EMR of only a narrowly specified range of wavelengths which can be separated into different colors. "We feel infrared light as heat and our radios pick up the messages encoded in radio waves emitted by radio stations. Ultraviolet light has high enough energy to damage our skin cells.... The special bulbs called 'black lights' produce a lot of UV...."

The full reach of the terminology becomes clear in the following passage. It begins by sorting out light as a specific "form of electromagnetic radiation", although the adjective "visible" is employed to characterize it. "The form of electromagnetic radiation your eyes can detect is called 'visible' or 'optical'." But then light is identified with the entire EM spectrum. "Astronomers have only recently (within the past few decades) been able to use the other forms of electromagnetic radiation or light." And the discussion closes by switching back and forth between 'electromagnetism' and 'light', i.e. they are being treated as meaning the same. "Every time technology has been developed to detect another form of light, a revolution in our understanding of the universe has occurred. The figure above shows all of the forms of electromagnetic radiation in order of increasing wavelength."

it should be noticed that the "figure above", the schematic presentation, is set out as being of 'The Electromagnetic Spectrum'. 'Light' is not given as one of the categories but 'Visible (Optical)' is. That is, the schema differs only in a minor detail from the previous text: a certain range of EMR is not labeled 'Light' but something we can surely take as equivalent, namely as visible or optical electromagnetic radiation.

So this text, while internally inconsistent in its use of the relevant terminology, tends to go further than the previous text in using the words 'light' and 'electromagnetic radiation' as equivalent.

A similar pattern can be found in other books.[16] An important phenomenon called 'electromagnetic radiation' has been discovered.  Light, that which affects our optical system and enables us to see, has been found to be a form of EMR.  It occupies a specified, rather modest, segment of the EMR spectrum. Other portions of the spectrum do not affect our eyes. [17]  However, before long,the texts provide 'light' with an adjective, 'visible'.  Soon it acquires further adjectives, first, 'invisible' and then 'ultraviolet' and 'infrared'.  The most radical movement awy from the starting place in which light is a modest slice of the EMR spectrum takes 'light' to be simply another word for 'eledctromagnetic radiation'.  

Churchland's account of EMT, which I have already described as mistaken, is not then his invention and it would be quite shortsighted to say or imply as much. Such texts as described above, especially one similar to Strobel's, are the background, even the source, of his way of conceiving of EMT.

However, learning about the origins of Churchland's account alters absolutely nothing in the earlier criticism of his view. Rather what is now apparent is that same criticism must be extended to the books, the scientific texts, which have influenced him.

That criticism was that there has been nothing in the theoretical or experimental development of EMT that shows, or even has a tendency to show, that, for instance, the infrared portion of the EMT spectrum is really light or that all electro-magnetic radiation is nothing but light.

The authors of the texts are in a state of uncertainty as to how to specify the place of light in the EM spectrum. Is it that portion which can be broken down into colors and to which our eyes are responsive? Or is it to be extended to some adjacent portions of the spectrum but not to all? Or should we say that the entire spectrum consists of nothing but light? The best explanation of that variation in locating light on the spectrum is that there is nothing in EMT itself that determines how we should talk.

One might even argue that there is only one discovery that would have shown that other segments of the spectrum beyond that falling between 400 and 700 nm are (really) light. That discovery would have been that our eyes are normally activated by any EM wavelength (or by say a specific portion of it outside of the 400-700 range.) Nothing of that sort has been discovered – in fact all the texts, and Churchland too, specifically disavow that that occurs.

If it is not entailed, or implied, by EMT that light consists of some portions of the EM spectrum beyond that which affects our optical systems, what has produced the ideas that light is electromagnetic radiation or that there is, say, ultraviolet light?

What has happened is that the relevant textbooks have come to talk, occasionally but not always, as if 'light' were to be thought of as covering larger and larger portions of the spectrum, sometimes even to the point of identifying it with the entire spectrum. In doing so, there is only linguistic innovation, not scientific discovery – innovation not in the slightest supported by any new understanding of the world and how it operates, not by any theory which has the consequence that the best way to understand the phenomena is to see light as larger and larger portions of the EM spectrum.

The pertinent question is 'Why, then, have the authors of the texts come to talk so, to extend the reach of the word "light" beyond where it is normally used?' No explanation is advanced in the texts. The creeping linguistic imperialism is not even noticed.

It must be assumed that the innovation is not undertaken solely on the part of the authors of the texts. Presumably the relevant scientific communities are the ones that have produced the verbal expansion and the authors of the texts are merely reflecting how the language of light is used within the discipline. It would also seem reasonable to think that there has not been disagreement within the disciplines about the innovation, otherwise the texts might well make some reference to it.

In the end, questions as to when the extension first appeared, when it became widely accepted, whether reasons were ever advanced for it, whether criticism was ever produced, etc. are questions for the history of science.[18]

In the absence of such studies, and I am guessing that they have not been undertaken, one can only speculate as to why the expansion occurred. The only reasonable guess, given a deprived empirical basis, is that it came about as a result of a certain kind of institutionalized or disciplinary intellectual sloppiness. What I have in mind is this. Within the sciences some terms are defined very carefully and narrowly. But terms derived from our common language are treated casually, as if they were subject to no constraints. It seems plausible that that was the fate of the term 'light' within the physical sciences.

What has happened here is not at all like the introduction of the terminology of 'flavors' in particle physics. That was a recognized bit of charming innovation where a term was redeployed to mark out a newly recognized group of entities. The development concerning the term 'light' is related to, though not identical with, what Stephen Jay Gould has noted: "In fact, this explicit denial of importance to modes of communication has, unfortunately, engendered a more than merely mild form of philistinism among many scientists who not only view verbal skills as unimportant...casting suspicion upon the writer's capacity for objectivity in presenting the data of nature. In an almost perverse manner, inarticulateness almost becomes a virtue as a collateral sign of proper attention to nature's raw empirics versus distilled human presentation thereof."[19]

To produce a reminder of how arbitrary that innovation has been, notice some very different linguistic developments that would have been equally possible though never so much as hinted at. Rather than speaking of 'ultraviolet light', it would have been equally possible, though grammatically more awkward, to talk of 'optically accessible ultraviolet waves', that is, to have made ultraviolet, rather than light, the fundamental category and then to talk as if light were just a form of ultraviolet. Or more radically 'electromagnetic' might have been treated as if it means the same as, say, 'gamma rays', so that all portions of the spectrum are (really) just 'forms of gamma rays' (or radio waves, etc.) All such manner of expression are as much justified, on the basis of the actual developments in science, as what has become the standard practice, that of the freewheeling expansion of the use of 'light'.

Let me put it the other way round. It would be possible for physics and astronomy to repress all references to 'infrared light', 'ultraviolet light', and the identification of light with the entire range of the electromagnetic spectrum and thereby not make a single change to EMT.

Churchland has, probably for reasons found in his own philosophical inclinations, tended to follow the most radical of the textbook innovations and talk as if 'EMR' and 'light' mean the same and that they have come to do so as a matter of scientific discovery. Sometimes he does resort to the more cautious line wherein only the infrared and ultraviolet portions of the spectrum are thought of as light. "And third, infrared and ultraviolet light are quite invisible to terrestrial eyes."[20] He might as well have said that of microwave radiation – but exercising caution in that place did not.

Is there any reason why scientists involved with EMT, and philosophers too, should resist the linguistic innovations? Would objections to those innovations be merely conservative, complaints about a change simply because it is something new?

There is reason to object. Consider the following from Churchland. "Our eyes evolved to exploit a narrow window of EM transparency in the Earth's idiosyncratic atmosphere and oceans. Nothing of ontological importance need correspond to what makes our rods and cones sing."[21] From that recognition one can construct an objection to the extension of the language of light as found in the scientific textbooks and in Churchland.

Our (including other species) eyes evolved in the conditions Churchland mentioned. They, as a matter of historical development, came to be organs that respond to ("sing") only to a certain narrow range of electromagnetic radiation. We humans came to talk of the phenomenon so experienced as light. Hence the concept of light is quite provincial. If one goes about broadening the concept, as is done in the textbooks, and ironically enough in Churchland, then one puts oneself in a position in which that important recognition about our evolutionary history may be unavailable. The "may be" is all that can be said: for if the talk of light as extending beyond the 400-700 nm range of EMR is not taken really seriously (as I shall argue later), then one can flip back to that understanding of light when it becomes important to make the above evolutionary point.

What is crucially taken for granted in my entire account of the proper place of light in EMT and by my charge that it is merely unsupported linguistic imperialism that has produced the current ways of talking is that the concept of light is an ordinary, not a scientific, concept. As a result of that rather obvious point, it is possible to recognize that physical investigation of the world has (had) as one of its aims to discover the physical nature of what we humans call 'light'. That has been (magnificently) accomplished by learning that it is a form of EMR that falls between 400-700nm in wavelength. That understanding gets hooked up with an evolutionary realization that we did not evolve organs that enable us to perceptually register other modalities of EMR.[22]

The contention being criticized in this paper, that the concept of light has been altered by scientific progress and understanding, is not intended to stop at only the scientific literature. For the point of the opening argument was to hold that our ways of talking outside of science have been altered by what has happened in the physical sciences. And that was taken to mean that the very concept of light has been altered by EMT.

I think that there is some truth to that claim. It is not just scientists and philosophers such as Churchland who now speak differently than they used to: lots of ordinary folk do too. The talk of 'infrared light' and 'ultra-violet light' and of 'visible' and 'invisible' light is not confined to an elite band of professionals and afficionados. That talk is understood and employed by a wider circle of human beings. And that is to be expected: lots of non-professionals who are taught from the texts, find that way of talking employed there, and come to take it outside the classroom and to spread it by various means.[23]

But that outcome is not at all what the original argument, and Churchland, had in mind. For that the concept of light has been changed and changed by developments in science was taken to be the result of discovery, of coming to know. We were to admire the role of science in this regard. But what turns out is that the power of science, in this case, to effect conceptual alteration is not derived from something admirable in science, but rather from erratic and unconstrained linguistic practices. So let's chalk up a tie: a conceptual change has come about as a result of scientific discovery but it has not come about in a manner that redounds to the credit of science.

I must point out that I said that the claim that there have been changes in the concept of light as a result of EMT has some truth in it. And I have tried in the previous paragraph to say what that truth is. Let me now set out what I meant by the qualification, that it has only some truth. It is here that the important preliminary work on what a concept is and what is involved in the concept of light would play a role. In the absence of that work what I have to say now has something of a dogmatic character.

To speak of a concept is to speak of what it does and does not make sense to say where we want to talk not about a word, which of course belongs to a particular language, but about words which mean the same no matter what language they are in. So, for example, to speak of the concept of belief is to talk about what it does and does not make sense to say of belief no matter whether we are talking of the English word 'belief' or whatever word in whatever language plays that role.

It should be noticed that both the opening argument and Churchland are implicitly depending upon precisely the view of concepts above, namely that what constitutes a concept is what it does and does not make sense to say. For both hold that it is the post-EMT intelligibility, meaningfulness, of the phrase 'invisible light' which establishes that there has been conceptual change.

The first point to notice is that what the concept of light comprises, on that account of concepts, is quite extensive: many different things concerning light do and do not make sense. In a more full discussion, I would here have run through an interesting sample of items involved in the concept of light, most of which are quite unaffected by anything related to EMT. At best only a small portion of the concept has been touched.

The original argument holds that we now have, after EMT, a "different concept". That is simply not so. If one is a Heraclitean or a Humean and have the idea that any change produces a new being, then any change in the concept of light would produce a new concept. But the conception of identity being relied upon there is not acceptable. Things change without being new things. The concept of light can undergo changes without becoming a different concept.

One might defend the opening argument by saying that what has changed in the concept of light is so central to the concept that we can legitimately speak of a different concept. So, it is alleged, previously "the explanation" of light prior to EMT was that light is "what makes things visible"; as we now sensibly, intelligibly, can talk of invisible light, it is no longer possible to offer the explanation 'light is what makes things visible'. It is being assumed (correctly) that the core of the concept prior to EMT is connected with visibility. Hence, with the new intelligibility of 'visible/invisible light' comes a new concept no matter what or how much else remains the same.

First, if the following conversation could now take place, then it would be necessary to grant that the core of the concept had changed and changed as a result of EMT.

X: 'Is it light out yet?'

Y: 'Let me check? [Does – looks out the window.] I can't see a thing – so there is no visible light. Still, there might be some microwave radiation about – you know the background stuff from the creation of the universe. And gamma rays – they say they're always about. I know! Let me check the radio. (Does so – twiddles the dial) Ah yes there are lots of radio waves around. So it is light out.'

If we had come to talk so, then there would have been a central change in the concept of light. But we have not come to so speak, think and act – not even, I'm sure, the astronomers and physicists who have encouraged talk of invisible light as well as those philosophers who champion the cause of invisible light.

Visibility has not "dropped out of the concept of light". It still is central to our ordinary practice, in the ways we talk, act and think. Rather what has happened is that there has been a minor addition to our talk – we now can sensibly speak in some contexts of visible and invisible light. But we continue to deny in and by our daily practice that there is light about when we cannot see anything. What we thus do is shield our normal talk about visibility requiring light from the newly intelligible phrase 'invisible light'. From the vantage point of the concept of light, what we have done is to put the new way of talk on a reservation or a Bantustan and preclude it from having any significant contact with our everyday thought and talk about light.

We can (sensibly), and probably still do, give the explanation 'Light is what makes things visible' although, if we were to think of it, we might add 'though we have also come to mention invisible light in some circumstances'.[24]

Hence the philosophical views I have been criticizing here go wrong on both major theses: it is not a scientific discovery that some light is invisible and that we have nonetheless come to talk of invisible light is not a radical revolution in the concept of light.

Let me set out a final difficulty with easy talk of a change in the concept of light.

Concepts are not objects, not entities – they are not, say, a something similar to a Platonic Form (a changeable Form?) Changing a concept is not like painting a mustache on La Giaconda. For not all languages may come to incorporate the change accepted  by other languages. So with concept of light: just because some speakers, some languages, have come to allow talk of 'invisible light' does not mean that all those everywhere do. In fact, it is quite easy to imagine that speakers in the highlands of New Guinea, say, would laugh off any attempt to tell them that light can be invisible. That certainly may not be because they are ignorant and primitive. Instead they might come to see us as gullible. If so, if only some group of languages does incorporate talk of the invisibility of light, should we say that the concept has changed? I don't know. However, it is first of all arrogant to think that science dictates what does and does not make sense to say and further arrogant of us, just because we have gone with the scientific flow, albeit in a minor way, to think that we have the authority over what counts as a feature of a concept. But then we are unlikely to be able to put aside this trouble with the idea of conceptual change until we have produced a generally acceptable account of what a concept is.


1. The argument here was originally formulated by my former colleague Brian Garrett, now at McMaster University, following a discussion between Paul Churchland, Meredith Williams and myself on how to practice philosophy. Since I do not know whether Garrett accepts the argument, I cannot attribute it to him.

2. See Jesse J. Prinz, Furnishing the Mind: Concepts and Their Perceptual Basis (The MIT Press, 2002) for a review of theories of concepts: the one I would defend, one derived from the Wittgensteinian tradition, is not even mentioned in his survey.

3. Andrew Fraknoi, David Morrison, Sidney Wolff, Voyages Through the Universe (Saunders College Publishing, 2nd Edition, 2000), p. 85.

4. ibid., p. 86.

5. The one piece of philosophical writing which has something to say about the issues addressed here is a piece by Paul Churchland entitled 'The Rediscovery of Light' published in Paul M & Patricia S. Churchland, On the Contrary: Critical Essays 1987-1997 (The MIT Press, 1998). Despite the title of, and despite the fact that embedded in it are some comments relevant to my project, Churchland's paper has a wholly different aim from mine. He there takes on a set of arguments, most of them by Searle (the title of the paper is of course a reference to Searle's The Rediscovery of Mind.) Those arguments come to the conclusion that consciousness is irreducible. Churchland's tactic is to take each of those arguments and attempt to show that it is fallacious by producing an appropriate analog argument concerning light. He argues that the analogs are known to be fallacious, since light has already been shown to be reducible. Hence we ought to regard Searle's arguments as fallacious. Here I am not at all interested in pursuing Searle on consciousness nor the adequacy of Churchland's models of Searlean arguments. I am interested only in what Churchland has to say, and often imply, about light as it occurs by-the-way in his pursuit of Searle on consciousness. Problems arise in discussing Churchland in connection with the present topic because his targets are so radically different from mine. I can only apologize in advance if I have been in any way unfair to him as a result.

6. 'The Rediscovery of Light', p. 140.

7. ibid., p. 132.

8. ibid., p. 129-130.

9. Although mistaken about the claim 'Light is EMR', Churchland is still entitled to a statement 'Light is EMR of between 400 and 700 nm'. He can treat that as an identity statement and continue making his objections to Searle, et al.

10. Voyages, p. 86.

11. loc cit.

12. loc cit.

13. loc cit.

14. ibid, p. 87.

15. Nick Strobel, Astronomy Notes (Primus/McGraw-Hill, 2004 edition, ISBN 0073122491). It is also published on the web at I have used the web edition here with the consequence that there are not page numbers. All the quoted material can be found in the (decently short) chapter 'Electromagnetic Radiation (Light)'.

16. See for instance R.K. Prinja & R. Ignace, Understanding the Universe (Checkmark Books, 2002). This is not a text for classroom use but a 'coffee table' text by two highly reputable astronomers. Its pattern of discussion of EMT is closer to the Strobel text than to Voyage Through the Universe.

17. Of course ultraviolet radiation too can affect our eyes: it can burn them. That was not what was intended though.

18. A historical note: the introduction of the phrase 'invisible light' preceded Clerk Maxwell's creation of EMT. William Herschel, while measuring the temperatures of the different color bands produced by sunlight passing through a prism, decided one day to put a thermometer outside the red light: and discovered that there was a temperature higher than ambient in that location. He wrote: "I conclude that the full red falls still short of the maximum of heat, which perhaps lies even a little beyond visible refraction. In this case, radiant heat will at least partly, if not chiefly, consist, if I may be permitted the expression, of INVISIBLE LIGHT: that is to say of rays coming from the sun, that have a momentum as to be unfit for vision." (Royal Society, Philosophical Transactions, 1800). I owe the reference to my colleague Stephen Simon. Herschel is credited today with the discovery of infrared radiation. He at least recognized that he was introducing new terminology and not simply marking a new discovery.

19. Stephen Jay Gould, The Hedgehog, The Fox and The Magister's Pox: Mending the Gap Between the Sciences and the Humanities (Random House, 2003), p 132.

20. Churchland, op. cit., p. 127.

21. loc. cit.

22. It is worth a note here to point out that while I have focused on EMR in the physical sciences, its place in the biological sciences also bears philosophical scrutiny. Biologists concerned with the perceptual systems of other animals have learned that "ultra-violet-sensitive receptors appear to be extremely common in insects, avians, fishes and reptiles, as well as being present in some mammals and amphibians." ('Limits to the Salience of Ultraviolet: Lessons from Colour Vision in Bees and Birds', P. Kevan, L. Chittka and A. Dyer, The Journal of Experimental Biology, 204, 2571-2580 (2001), p. 2572. That piece is a fairly comprehensive review article.) Biologists have found that in such creatures as mentioned there are, embedded in the eyes, specific receptors which are activated by ultra-violet radiation (receptors analogous to rods and cones.) However, they have been led by that discovery to talk as if ultra-violet were another color. "The relative importance of ultraviolet vis a vis other primary colours...." (loc cit., p. 2571 and passim.) Of course ultra-violet is not a color – it is, for instance, not included in color charts, etc. So what we find in the biological writings is similar to what is found in the physical writings about light: an expansion of the meaning of a term which is not justified by the phenomena discovered. On the other hand, why ultra-violet has come to be talked of as a color is readily discernible: it is that ultra-violet radiation has its receptors in the eyes, along with receptors for light. Now that has correctly led to the talk of ultra-violet as a visual phenomenon for such creatures – it is, however, quite another step to include ultra-violet among colors. For color, as well as light, is a parochial concept, relating to human visual system and human visual language.

23. The technology of using bulbs similar to ordinary light bulbs and of infrared photography and night vision goggles has contributed as much as EMT to our talk of ultraviolet and infrared light. No such identification can account for the identification of light with EMR.

24. I have a further and perhaps idiosyncratic complaint. The terminology of 'visible/invisible light' is misguided in yet another way. For light is not visible. When we look outside to see whether it is light yet, we look to discover whether we see any trees, cows, cars, clouds, the sky, etc. We do not see light – though we might see the sun or a campfire or a light bulb, e.g. a source of light. If one takes a preliminary tour through the concept, it seems that the focus of our talk of light – of what it does and does not make sense to say - is on seeing sources of light, not light itself. Hence, light is neither visible nor invisible – just as water is neither wet nor dry but is that which makes things wet no matter what the temptation on the part of some people to think of it as the paradigm wet substance. The only piece of talk I know which may run counter to this is 'We can see the light at the end of the tunnel'. Whether that is a genuine contrary case would require further investigation into the concept of light.