The Intellect Versus the Senses
Table of Contents
How is light produced?
Light is usually produced by electrons in an atom where they ‘do something’ around the nucleus.
An electron and proton have no color.
But their union in the hydrogen atom produces electro-magnetic radiation of a certain discrete array of wavelengths.
The homogeneous constituents of this radiation, when separated by a prism or an optical grating, stimulate in an observer the sensations of red, green, blue, violet by the intermediary of certain physiological processes, whose general character is sufficiently well known to assert that they are not red or green or blue, in fact that the nervous elements in question display no colour in virtue of their being stimulated.
The white or grey the nerve cells exhibit whether stimulated or not is certainly insignificant in respect of the colour sensation which, in the individual whose nerves they are, accompanies their excitation.
Yet our knowledge of the radiation of the hydrogen atom and of the objective, physical properties of this radiation originated from someone’s observing those coloured spectral lines in certain positions within the spectrum obtained from glowing hydrogen vapour. This procured the first knowledge, but by no means the complete knowledge.
To achieve it, the elimination of the sensates has to set in at once, and is worth pursuing in this characteristic example.
The colour in itself tells you nothing about the wave-length; in fact we have seen before that, for example, a yellow spectral line might conceivably be not ‘monochromatic’ in the physicist’s sense, but composed of many different wave-lengths, if we did not know that the construction of our spectroscope excludes this. It gathers light of a definite wave-length at a definite position in the spectrum. The light appearing there has always exactly the same colour from whatever source it stems.
Even so the quality of the colour sensation gives no direct clue whatsoever to infer the physical property, the wave-length, and that quite apart from the comparative poorness of our discrimination of hues, which would not satisfy the physicist.
A priori the sensation of blue might conceivably be stimulated by long waves and that of red by short waves, instead of the other way round, as it is.
To complete our knowledge of the physical properties of the light coming from any source a special kind of spectroscope has to be used; the decomposition is achieved by a diffraction grating.
A prism would not do, because you do not know beforehand the angles under which it refracts the different wave-lengths. They are different for prisms of different mat- erial. In fact, a priori, with a prism you could not even tell that the more strongly deviated radiation is of shorter wave-length, as is actually the case.
The theory of the diffraction grating is much simpler than that of a prism.
From the basic physical assumption about light - merely that it is a wave phenomenon - you can, if you have measured the number of the equidistant furrows of the grating per inch (usually of the order of many thousands), tell the exact angle of deviation for a given wave-length, and therefore, inversely, you can infer the wave-length from the ‘grating constant’ and the angle of deviation.
In the Zeeman and Stark effects, some of the spectral lines are polarized.
To complete the physical description in this respect, in which the human eye is entirely insensitive, you put a polarizer (a Nicol prism) in the path of the beam, before decomposing it; on slowly rotating the Nicol around its axis certain lines are extinguished or reduced to minimal brightness for certain orientations of the Nicol, which indicate the direction (orthogonal to the beam) of their total or partial polarization.
Once this whole technique is developed, it can be extended far beyond the visible region. The spectral lines of glowing vapours are by no means restricted to the visible region, which is not distinguished physically. The lines form long, theoret- ically infinite series.
The wave-lengths of each series are connected by a relatively simple mathematical law, peculiar to it, that holds uniformly throughout the series with no dis tinction of tha t part of the series that happens to lie in the visible region. These serial laws were first found empirically, but are now understood theoretically.
Naturally, outside the visible region a photographic plate has to replace the eye. The wave-lengths are inferred from pure measurements of lengths: first, once and for all, of the grating constant, that is the distance between neighbouring furrows (the reciprocal of the number of furrows per unit length), then by measuring the positions of the lines on the photographic plate, from which, together with the known dimensions of the apparatus, the angles of deviation can be computed.
These are well-known things, but I wish to stress two points of general importance, which apply to well-nigh every physical measurement.
The state of affairs on which I have enlarged here at some length is often described by saying that, as the technique of measuring is refined, the observer is gradually replaced by more and more elaborate apparatus. Now this is, certainly in the present case, not true; he is not gradually replaced, but is so from the outset.
I tried to explain that the observer’s colourful impression of the phenomenon vouchsafes not the slightest clue to its physical nature.
The device of ruling a grating and measuring certain lengths and angles has to be introduced, before even the roughest qualitative knowledge of what we call the objective physical nature of the light and of its physical components can be obtained.
This is the relevant step. That the device is later on gradually refined, while remaining essentially always the same, is epistemologically unimportant, however great the improve- ment achieved.
The second point is that the observer is never entirely replaced by instruments; for if he were, he could obviously obtain no knowledge whatsoever.
He must have constructed the instrument and, either while constructing it or after, he must have made careful measurements of its dimensions and checks on its moving parts (say a supporting arm turning around a conical pin and sliding along a circular scale of angles) in order to ascertain that the movement is exactly the intended one.
True, for some of these measurements and check-ups the physicist will depend on the factory that has produced and delivered the instrument; still all this information goes back ultimately to the sense perceptions of some living person or persons, however many ingenious devices may have been used to facilitate the labour.
The observer must, in using the instrument for his investigation, take readings on it, be they direct readings of angles or of distances, measured under the microscope, or between spectral lines recorded on a photographic plate.
Many helpful devices can facilitate this work, for instance photometric recording across the plate of its transparency, which yields a magnified diagram on which the positions of the lines can be easily read.
But they must be read! The observer’s senses have to step in eventually. The most careful record, when not inspected, tells us nothing.
So we come back to this strange state of affairs. While the direct sensual perception of the phenomenon tells us nothing as to its objective physical nature (or what we usually call so) and has to be discarded from the outset as a source of information, yet the theoretical picture we obtain eventually rests entirely on a complicated array of various informations, all obtained by direct sensual perception.
It resides upon them, it is pieced together from them, yet it cannot really be said to contain them. In using the picture we usually forget about them, except in the quite general way that we know our idea of a light-wave is not a haphazard invention of a crank but is based on experiment.
I was surprised when I discovered for myself that this state of affairs was clearly understood by the great Demo- critus in the fifth century B.C., who had no knowledge of any physical measuring devices remotely comparable to those I have been telling you about (which are of the simplest used in our time).
Galenus has preserved a fragment where Democritus introduces the intellect having an argument with the senses about what is ‘real’.
There is colour, sweetness, bitterness, actually only atoms and the void
Poor intellect, do you hope to defeat us while you borrow from us your evidence? Your victory is your defeat.
I contrast 2 general facts:
- All scientific knowledge is based on sense perception
- The scientific views of natural processes formed in this way lack all sensual qualities and therefore cannot account for the latter
Scientific theories serve to facilitate the survey of our observations and experimental findings.
Every scientist knows how difficult it is to remember a moderately extended group of facts, before at least some primitive theoretical picture about them has been shaped.
It is therefore small wonder, and by no means to be blamed on the authors of original papers or of text-books, that after a reasonably coherent theory has been formed, they do not describe the bare facts they have found or wish to convey to the reader, but clothe them in the terminology of that theory or theories.
This procedure, while very useful for our remembering the facts in a well-ordered pattern, tends to obliterate the distinction between the actual observations and the theory arisen from them.
Since the former always are of some sensual quality, theories are easily thought to account for sensual qualities; which, of course, they never do.