First and Last Things

First and Last Things

4. Skepticism of the Instrument

At first I took the world of fact as being exactly as I perceived it. I believed my eyes. Seeing was believing, I thought. Still more did I believe my reasoning. It was only slowly that I began to suspect that the world of fact could be anything different from the clear picture it made upon my mind.

I realized the inadequacy of the senses first. Into that I will not enter here. Any proper text book of physiology or psychology will supply a number of instances of the habitual deceptions of sight and touch and hearing. I came upon these things in my reading, in the laboratory, with microscope or telescope, lived with them as constant difficulties. I will only instance one trifling case of visual deception in order to lead to my next question. One draws two lines strictly parallel; so

Oblique to them one draws a series of lines; so a series of parallel and closely-spaced lines drawn through each horizontal line, top sloping to the right, bottom to the left

a series of parallel and closely-spaced lines drawn through each horizontal line, top sloping to the right, bottom to the left

and instantly the parallelism seems to be disturbed. If the second figure is presented to any one without sufficient science to understand this delusion, the impression is created that these lines converge to the right and diverge to the left. The vision is deceived in its mental factor and judges wrongly of the thing seen.

In this case we are able to measure the distance of the lines, to find how the main lines looked before the cross ones were drawn, to bring the deception up against fact of a different sort and so correct the mistake. If the ignorant observer were unable to do that, he might remain permanently under the impression that the main lines were out of parallelism.