Ibn al-Haytham (ca. 965 – ca. 1040), known to the Latin practitioners of perspectiva as Alhazen or Alhacen, has been called the father of modern optics. The well-documented Wikipedia article says that his seven-volume Book of Optics, which builds on Ptolemy, Euclid, Galen, and Aristotle, was frequently cited by Kepler, Galileo, Huygens, and Newton. I won’t dwell here on his scientific contributions, which were many; what I find especially interesting is his clear articulation of what Helmholtz in the 19th century would call “unconscious inference” in sense perception.
A notable advocate of critical thought, Ibn al-Haytham wrote that “I constantly sought knowledge and truth, and it became my belief that for gaining access to the effulgence and closeness to God, there is no better way than that of searching for truth and knowledge” (quoted, Wikipedia).
“Truth is sought for its own sake … Finding the truth is difficult, and the road to it is rough. For the truths are plunged in obscurity…. God, however, has not preserved the scientist from error and has not safeguarded science from shortcomings and faults. If this had been the case, scientists would not have disagreed upon any point of science” (ibid).
“[T]he seeker after the truth is not one who studies the writings of the ancients and, following his natural disposition, puts his trust in them, but rather the one who suspects his faith in them and questions what he gathers from them, the one who submits to argument and demonstration, and not to the sayings of a human being whose nature is fraught with all kinds of imperfection and deficiency. The duty of the man who investigates the writings of scientists, if learning the truth is his goal, is to make himself an enemy of all that he reads, and … attack it from every side. He should also suspect himself as he performs his critical examination of it, so that he may avoid falling into either prejudice or leniency” (ibid).
“From the statements made by the noble Shaykh, it is clear that he believes in Ptolemy’s words in everything he says, without relying on a demonstration or calling on a proof, but by pure imitation (taqlid); that is how experts in the prophetic tradition have faith in Prophets, may the blessing of God be upon them. But it is not the way that mathematicians have faith in specialists in the demonstrative sciences” (ibid).
José Filipe Silva in “Perceptual Judgement in Late Medieval Perspectivist Psychology” (2017) writes that “Alhacen has an instrumental approach to faculty psychology, in the sense that he is interested in providing an account of visual perception in terms of functions and mechanisms, rather than in terms of faculties. In that sense, he causes a problem to his medieval interpreters who operate … under the framework of Avicennian faculty psychology” (pp. 1-2).
“A primary concern of late medieval philosophy is how things are made available to perceivers in such a way that they are perceived in an accurate manner. Because things cannot be themselves immediately present to the senses, one needs to posit some form of representation that makes things available. Two issues follow from this: the first concerns the nature of these representations, in terms of their power to represent (what they represent), and the second their ontological status in the medium and in the senses, i.e., the kind of existence or being they have” (p. 2).
“[S]ome late medieval authors seem to have become aware of the limitations of an account of cognition that allow us, as finite beings, to build accurate representations of the external world and its objects on the basis of (the processing of) incoming sensory information by our sensory faculties…. Perspectivist optics tries to address these concerns by strengthening the process of producing and certifying the final product, the image of the external thing acquired by visual perception, by rational-like processes — namely by judgment and inference” (p. 3).
” ‘[D]ifferentiation between two greens is not the actual sensation of green’. But this is still perception by sight; or, better, it is a case of seeing (‘it occurs in sight’) while not being ‘the sensation of colour’…. [T]his visual property can only be perceived by comparison (per comparationem) and discrimination (per distinctionem). According to Alhacen, such an operation is accomplished by what he calls the power of discrimination, the virtus distinctiva. The important and original claim is that any instance of visual experience consists of both the perception of the form that is seen and the further act of discrimination, which is the perceptual judgment, e.g., of comparison. A basic distinction is then at play between: (i) perception at first sight (comprehensio solo sensu) [and] (ii) perception by judgment (comprehensio per distinctionem/cognitionem/scientiam)” (pp. 4-5).
I think such a view is also implicit in Aristotle. What distinguishes Ibn al-Haytham in this regard is that he makes it very explicit.
“There is another function of the faculty of discrimination that resonates to a contemporary mind: it can recognize the perceived object without having to go through all its characteristics, provided it has previously encountered that thing…. Alhacen therefore introduces yet another level: (iii) perception by means of reasoning (comprehensio per argumentationem/sillogismum). According to this last type, perception in the robust sense, i.e., as the perception of all properties/intentiones constituting the sensible form, must include what has often been called (unconscious) sensory inference, because the perception of some of those properties is dependent on acquired knowledge and presupposes a process akin to reasoning: the immediate grasping of a conclusion that follows from the premises without knowing the relation of entailment between premises and conclusion. Alhacen notes that, even though structurally it operates in a quasi-rational way, such perception does not qualify as cognition in the full rational sense, because it is not linguistic (it does not make use of words)” (pp. 5-6, citations omitted).
“Once it possesses these forms in its imagination and encounters similar instances of the same kind, or the same individual, the soul performs what Alhacen calls the second type of perceptual intuition, which is perceptual intuition with previous knowledge. In these cases, Alhacen describes how cognition or perception takes place when the form which is being perceived is compared with the form which is stored in the imagination, namely to its similarity to a general or an individual form already acquired. If it ‘fits’/corresponds to the universal form, the cognitive power of discrimination identifies the kind to which the individual now perceived belongs, whereas if it bears correspondence with an individual form, it recognizes the individual thing…. But the process is often swifter, because the power of discrimination is able to recognize an individual or a kind on the basis of distinctive or salient features” (p. 6).
“In other words, before one knows what a thing (‘red’) is, one perceives the difference between that thing and other things, i.e., the difference between ‘red’ and ‘blue’; once the knowledge of ‘red’ has been acquired, one begins to immediately see ‘red’ (quod est color, insofar as it is colour, an instance of perception at first sight) followed by the recognition of ‘red’ as the kind of colour it is (cuiusmodi sit color or the quiddity of the colour red)” (ibid).
“All this is done in an amazingly short time, especially in the case of perception at first sight. In the case of perception by judgement and reasoning, which are slower than perception at first sight, the process is faster if the objects are familiar (‘frequently perceived’) to the perceiver. In this case, the perceiver has a form retained in his/her memory to which it has access, and that can be applied to the identification of the thing present to the senses, rather than having to go through the process of discriminating all the intentions that constitute the object’s sensible form. As Alhacen makes clear, this is possible due to the way these properties are made available and the ‘familiarity’ of the power of discrimination with them. But this comes at a cost, as it means that it can make mistakes, as recognition is a step removed from the actual seeing of the visual form, and is dependent on a complex combinatory process” (p. 7).
“[I]f a thing is well-known by the perceiver, some salient properties are enough for its identification and recognition. If, however, that is not the case, and the thing is unknown, the perceptual system – sensory power plus last sensor plus power of discrimination – must act on the entire spectrum of sensory information in order to unveil all of its intentions or sensible properties. Alhacen calls this perceptual intuition (per intuitionem) or ‘visual scrutiny’. Perceptual intuition is therefore the perception of the form of the visible thing with all its properties that includes discrimination and inference. In order to do so, i.e., to get a better hold of the object, the sensitive power will move the organ of sense to see the object from other viewpoints. This scanning process is automatically initiated as the result of the way the visual system is built (natus est visus)” (ibid).
“Finally, this allows also for a conclusion concerning the active nature of the perceptual process: if it were passive, it would simply be perception at first sight, just receiving the impressions of light and colour. As we can conclude from Alhacen’s arguments, it is not. Perception of the object’s visual form (the assemblage of its properties or intentions) is the result of complex and complementary levels of psychological functions, including discrimination, recognition, and inference” (p. 9).
The Postmodern Peripatetic 