THE OPTICS OF IBN AL-HAYTHAM

BOOK I ON THE MANNER OF VISION IN GENERAL

CHAPTER 1: PREFACE TO THE [WHOLE] BOOK

Early investigators diligently pursued the inquiry into the manner of visual sensation and applied their thoughts and effort to it, eventually reaching the limit to which their investigation had led, and gaining as much knowledge of this matter as their inquiry and judgement had yielded.

Nevertheless, their views on the nature of vision are divergent and their doctrines regarding the manner of sensation not concordant. Thus, perplexity prevails, certainty is hard to come by, and there is no assurance of attaining the object of inquiry. How strong, in addition to all this, is the excuse for the truth to be confused, and how manifest is the proof that certainty is difficult to achieve!

For the truths are obscure, the ends hidden, the doubts manifold, the minds turbid, the reasonings various; the premisses are gleaned from the senses, and the senses (which are our tools) are not immune from error.

The path of investigation is therefore obliterated and the inquirer, however diligent, is not infallible. Consequently, when inquiry concerns subtle matters, perplexity grows, views diverge, opinions vary, conclusions differ and certainty becomes difficult to obtain.

Our subject is obscure and the way leading to knowledge of its nature difficult; moreover, our inquiry requires a combination of the natural and the mathematical sciences.

It is dependent on the natural sciences because vision is one of the senses and these belong to natural things.

It is dependent on the mathematical sciences because sight perceives shape, position, magnitude, movement and rest, in additionto its being characterized by straight lines; and since it is the mathematical sciences that investigate these things, the inquiry into our subject truly combines the natural and the mathematical sciences.

Natural scientists have inquired into the nature of this subject according to their art and exerted themselves in it as much as they could. The learned among them settled upon the opinion that vision is effected by a form which comes from the visible object to the eye and through which sight perceives the form of the object.

Mathematicians, for their part, have paid more attention to this science than others. They have pursued its investigation, paying attention to its details and divisions. They have distinguished objects of vision, assigning causes to their particular properties and stating reasons for each of them. All the same, they have continued throughout the ages to disagree about the principles of this subject, with the result that the opinions of the various groups among the practitioners of this art have gone different ways.