Thursday 30 June 2022

Lecture 8: The Space Enigmas I: Berkeley (Turvey, 2019, Lectures on Perception)

One of the big problems that emerges from all the proceeding discussions of perception is how we are able to perceive space. Space has been considered as a mathematical concept (in terms of Euclidean geometry), as a psychological concept (a construction of the mind) but never really as a biological, ecological concept. This first chapter about space perception is focused on one mathematical conception, some of it's implications, and one specific attempt to deal with those implications (Berkeley's New Theory of Vision). 

Space perception has to work with whatever space is, so a theory of space perception has to work with whatever your theory of space is. Mathematically, space has been considered as Euclidean - continuous, isotropic (no preferred direction), homogeneous, and infinite. (ADW note: there are lots of kind of geometrical systems, and they vary in how many of these kinds of symmetries they require. Affine geometries give up isotropism, for example, while topology relaxes all symmetries. So a mathematical conception of space for perception to work with does not have to be Euclidean, it just mostly has been considered as such. Identifying the correct geometry for perception is actually an empirical question, e.g. Todd et al, 2001, and there are plenty of options that may be weird enough.)

The Euclidean conception of space poses some problems for visual space perception to solve. In essence, vision inhabits Flatland, and faces all the challenges of coming to grips with Spaceland detailed in Abbott's famous book. In the first story, a 2D square living in Flatland tries but fails to convince a 1D inhabitant of Lineland that the 2nd dimension exists; in the next two stories. a visitor from 3D Spaceland tries to convince the square that the 3rd dimension exists, which only happens with the 'miracle' of the square being lifted into Spaceland. This convinces the square, who goes on to reason that there may be a 4th dimension; he cannot convince the sphere from Spaceland that this reasoning works, however. 

The relevant moral of the story is that there is nothing essential (inherent) to Flatland than can provide access to Spaceland, so you cannot get from one to the other using logic or analysis on Flatland data. The implication is that, in order to perceive space, Flatland vision needs help from a Spaceland inhabitant, which might come in one of two guises:

  1. You could enrich 2D vision with a source of knowledge about the rules of Spaceland geometry. This is the Kantian style solution, but is an example of an unrepayable loan of intelligence
  2. You could ground 2D vision with a perceptual system that necessarily inhabits 3D Spaceland, such as touch: this is Berkeley's solution, because he was worried about the loan, and which Turvey spends the rest of the Lecture on. 
(A third possible solution (binocular vision) gets ruled out by Turvey, who points out that a) most animals don't have overlapping fields of view and b) even amongst those that do, like humans, monocular vision persists as being successfully spatial.)

Berkeley actually gets rid of a lot of 2D cues; for him, what strikes the retina are sets of points of light with varying origins from a 3D environment. Vision consists of a bunch of contingent facts about these bundles; this set is not a rich set and certainly cannot support getting to Spaceland via computation on these sensations. In fact, for Berkeley, these facts operate exactly like a language: a series of arbitrary signs and symbols that don't work by resembling what they represent, but by virtue of being structured by a reasonably coherent set of reliable, regular relations. Just like language (or any symbol system) vision still needs to be grounded in order to be about anything. For Berkeley, that grounding comes from being associated with the sense of touch. 

Touch has long been considered by mechanist-based science to be a privileged system: it is about mechanical contact which can support local causation, so unlike vision it could operate as a direct perception system, no mediation required. Empirical work Turvey reviews does support the basic idea that the activity of the body (e.g. locomotion) can serve as an invariant source of information about distance and speed despite variable activity - blind walking & running from A to B can both support successful blind walking from B to A, for example. 

This seems to support the Berkeley idea, but Turvey very quickly shuts this down. While bodily movement does seem to serve as an invariant source of spatial information, it does so despite variable activity of the relevant sensors - this is the problem with vision that touch/haptics is supposed to be solving! Turvey notes here that Berkeley gives the final word to God, which is a hint he didn't really have a solid account. 


One thing that occurred to me is that this last point is quite important: it suggests that touch is not, in fact, a privileged, 'incorrigible' perception system because it has all the same issues as vision. So the common intuition that mechanical contact with things is somehow special and crucial to grounding perception is wrong. Berkeley was right, though, that perception needs to sit on a direct system at some point, but he didn't have the conceptual tools necessary to make it work, even for touch. This is going to be Gibson's critical contribution: providing a theory of specifying information that will enable direct perception to be a coherent option and thus make perception itself possible.

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