1.4.  Scepticism 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 realised 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

(two horizontal and parallel lines.)

Oblique to them one draws a series of lines; so

(a series of parallel and closely-spaced lines drawn through each horizontal line, one series (top) sloping to the right, the other (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.  And all the infirmities of eye and ear, touch and taste, are discovered and checked by the fact that the erroneous impressions presently strike against fact and discover an incompatibility with it.  If they did not we should never have discovered them.  If on the other hand they are so incompatible with fact as to endanger the lives of the beings labouring under such infirmities, they would tend to be eliminated from among our defects.

The presumption to which biological science brings one is that the senses and mind will work as well as the survival of the species may require, but that they will not work so very much better.  There is no ground in matter-of-fact experience for assuming that there is any more inevitable certitude about purely intellectual operations than there is about sensory perceptions.  The mind of a man may be primarily only a food-seeking, danger-avoiding, mate-finding instrument, just as the mind of a dog is, just as the nose of a dog is, or the snout of a pig.

You see the strong preparatory reason there is in this view of life for entertaining the suppositions that:—­