Do we see the world as it is? Ibn al-Haytham on vision

Look at the expectation: it asks you to understand the question about perception. Give an example from your own life where you saw something and took it for something else, and say what went wrong: the sensing or the interpreting?

Read one distinction closely. The record of Ibn al-Haytham's theory of vision reports that Smith, the modern editor and historian of the Book of Optics, finds in it a difference between sensing light and colour and inferring size and distance ². A tall man's representation in the perceptual apparatus is not itself as tall as that man, because interpretation, discrimination and memory intervene ². Light and colour are immediately sensed in this division, while distance, size and corporeity require comparison and experience ². The record gives a modern paper example: a width of 2 at distance 10 and a width of 4 at distance 20 subtend the same angle, which illustrates that angle alone does not give true size ². It adds that the example is not from a manuscript. Ask what the eye receives, and what the observer adds.

The records preserve no statement by Ibn al-Haytham of why he built this account, and Smith warns that its place in the development of his thought cannot be dated precisely ³. What they show is the problem and the field. Smith compares Aristotle, Galen, Ptolemy and Hunayn, identifying agreements as well as disagreements, and the record does not say which of them held outgoing rays ⁴. The challenge was to connect ray geometry with an affected bodily organ: geometry can order directions, but an explanation must account for coherent colour and shape rather than an indiscriminate jumble ⁵.

Smith warns against reading the account as modern neuroscience: he is analysing historical conceptual language, not attributing neurons or contemporary image processing to the author ². The record adds that it is not a retinal-image theory, since Alhacen located sensitivity in the crystalline humour ⁶. Nor is it a story in which accepting incoming light settles everything. The argument against outgoing visual rays asks why outward reach followed by return is necessary if incoming influence suffices, which the record calls an argument about explanatory redundancy, not evidence that every anatomical detail passed an independent test ⁴. Smith also shows that the idealised geometry of the eye serves ray selection rather than following anatomy ⁷.

The record rests on selected passages of Smith's 2001 introduction and on the TDV encyclopaedia, and states that no complete critical Latin text, English translation or fresh reading of the Arabic original is claimed ⁸. Smith is a modern editor and historian studying the Latin version, not a coauthor of the Arabic work ⁹. He names Kamal al-Din al-Farisi's commentary as an important vehicle of Arabic reception ¹⁰. The record also says that every manuscript, Latin copy and commentary was not examined ¹¹.

Smith says that Ibn al-Haytham separates sensing light and colour from inferring size and distance. Take a position on whether we perceive the world as it really is, using the example of the two widths at two distances. What is the strongest objection to your position?

• Ibn al-Haytham's theory of vision: the conditions of seeing, the model of the eye and its stated limits.
• Ibn al-Haytham: his life and works, and the question of whether the sources describe one man or two.
• Kitab al-Manazir: the book in which the account is set out.