Pointer readings ...

Eddington and friends

From Sir Arthur Eddington's The Nature of the Physical World:

'Let us then examine the kind of knowledge which is handled by exact science. If we search the examination papers in physics and natural philosophy for the more intelligible questions we may come across one beginning something like this: "An elephant slides down a grassy hillside…" The experienced candidate knows that he need not pay much attention to this; it is only put in to give an impression of realism. He reads on: "The mass of the elephant is two tons." Now we are getting down to business; the elephant fades out of the problem and a mass of two tons takes its place. What exactly is this two tons, the real subject matter of the problem? It refers to some property or condition which we vaguely describe as "ponderosity" occurring in a particular region of the external world. But we shall not get much further that way; the nature of the external world is inscrutable, and we shall only plunge into a quagmire of indescribables. Never mind what the two tons refers to; what is it? How has it actually entered in so definite a way into our experience? Two tons is the reading of a pointer when the elephant was placed on a weighing-machine. Let us pass on. "The slope of the hill is 60˚." Now the hillside fades out of the problem and an angle of 60˚ takes its place. What is 60˚? There is no need to struggle with mystical conceptions of direction; 60˚ is the reading of a plumb-line against the divisions of a protractor. Similarly for the other data of the problem. The softly yielding turf on which the elephant slides is replaced by a coefficient of friction, which though perhaps not directly a pointer reading is of kindred nature. No doubt there are more roundabout ways used in practice for determining the weights of elephants and the slopes of hills, but these are justified because it is known that they give the same results as direct pointer readings.' 

'And so we see that the poetry fades out of the problem, and by the time the serious application of exact science begins we are left with only pointer readings. If then only pointer readings or their equivalents are put into the machine of scientific calculation, how can we grind out anything but pointer readings? But that is just what we do grind out. The question presumably was to find the time of descent of the elephant, and the answer is a pointer reading on the seconds’ dial of our watch.' 

'The triumph of exact science in the foregoing problem consisted in establishing a numerical connection between the pointer reading on the weighing-machine on one experiment on the elephant and the pointer reading on a watch in another experiment. And when we examine critically other problems of physics we find that this is typical. The whole subject matter of exact science consists of pointer readings and similar indications' (p. 127).