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Gibson vs Physics: Gibson Wins, at the Ecological Scale

One of the interesting questions that popped out of our discussions with Ken Aizawa about Runeson and the Ames Room is this: did Gibson and his followers banish physics and geometry from his psychology? And if so, is Runeson breaking this prohibition by talking geometrically about the Ames Room? And how can you banish physics anyway - what the hell?

I think I've convinced Ken that there is no such prohibition in the comments at the posts linked to above (I think). But it's a topic of fairly central importance to the ecological approach, so I wanted to summarise some of these ideas and examples here.

(Psycho)physics

Physics describes objects in the abstract, as approximations in units that aren't relative to an observer. Remember, in the joke, it's the physicist who assumes the cow is a sphere. But we tend to treat the physical description as 'the way things actually are', the objective yardstick against which all science should be evaluated. Indeed, the science of psychophysics is explicitly premised on the idea that the job of the sensory systems is to convert the objects of the physical world into the objects of psychological experience, because the former are 'what there is to be known'. Vision science is therefore fascinated by illusions because they are supposed to reveal how our psychological experience often fails to correspond with the physical reality, due to the assumptions we have to impose in order to convert from the one to the other.

But recall the ecological response to illusions; first, they are generally readily revealed by exploration that breaks the limitation creating the problem. Second, and most relevantly here, they are only 'failures' if you think the job of the sensory systems is to bring your psychological experience into correspondence with an external world as described by the science of physics. It's precisely this that Gibson and his followers reject.

Gibson (1979)

Gibson does indeed spend quite a bit of time railing against physics; specifically, against the argument that the objects of perception are best described using the language of physics. Gibson spends the first three chapters of the 1979 book defining what things like 'surface', and 'ambient light' mean to an organism, rather than to physics. Sabrina has covered this in her Reading Group posts on Chapter 1, Chapter 2 & Chapter 3. These chapters are unique to Gibson (1979); no other perceptual psychologist feels the need to (re)define what the world is made of, given that this is what physics is supposedly for. I find them fascinating, precisely because no-one else thought it needed doing, while Gibson clearly felt he had to, in order to get his theory up and running. This is the first hint that Gibson is truly proposing something different.

Turvey, Shaw, Reed & Mace (1981)

But Gibson is not denying that information is rooted in physics; he's simply pointing out that the correct level of analysis for the information available to a perceiving organism is ecological. What an organism needs to know is not how far away something is, but whether it can reach that thing - in other words, affordances. You can't get to affordances via the objects of physics, because affordances are personal and contain meaning while physics is neither of these. You therefore have to get to affordances by detecting information about affordances, not units of physics.

The Establishment concern with this was summed up in the Fodor & Pylyshyn (1981) paper which I've discussed before as 'the dick move'. The dick move is to simply say this information could be trivial; if you detect the property of 'being a shoe' you will of course perceive a shoe. The reply by Turvey et al (1981) is the 'ecological laws' paper that lays out how information can arise via lawful relations, and how this information will therefore specify affordance properties within the scope of the law. (Ken is blogging about this paper here, and given how that's going I think I'll post about that paper in some detail again.) 'Being a shoe' isn't a candidate property because it doesn't lawfully give rise to a specific pattern in a perceptual array which an organism might detect.

So where does this leave physics?

Physics presumes a privileged, third-person perspective and describes things from this position. The science of perception is interested in what it is possible for an individual to know about, and the mechanism supporting that knowledge. Individuals do not have access to such a third person perspective, and thus the description of the world from that point of view is not appropriate for describing perception. Individuals exist in niches, with a specific set of capacities and opportunities for behaviour - these are what perception must be about, because these are what we must remain in contact with in order to behave. Gibson proposed that a) perception is of affordances, and occurs b) by virtue of information. Ecological optics is an attempt to scientifically engage with the topic of information, and is therefore itself an abstraction, like physics. However, it is an abstraction of the ecological level, and thus remains a more appropriate abstraction than physics.

Physics (and geometry) still have a key role to play, however. Ecological laws must obey the laws of physics. Information (spatial-temporal patterns in energy arrays) must arise from interactions between energy and surfaces and these interactions will be describable according to the laws of physics. The systems for detecting these patterns will evolve in line with the physical constraints of detecting that energy. Geometry is still an excellent tool for describing the resulting information, because geometry is concerned with form, shape and location and provides tools for discussing these qualities. There is more than one geometry, of course; the Euclidean geometry we're all familiar with from high school is just one of many, each making different assumptions about how form is preserved (or not) over various transformations. Euclidean geometry, for instance, is not a particularly good description of reach space; a better description is affine geometry (e.g. Todd et al, 2001).

Physics and ecological optics are abstractions whose purpose is to aid scientific analysis. Mathematics (especially geometry) is a system for formally interacting with these abstractions during that analysis. Like all tools, you either have the right one for the job, or you don't. The science of perception has to account for how individual organisms, who are situated in specific niches and who have specific evolutionary trajectories related to that niche, can come to have knowledge about the possibilities for behaviour within that niche. Physics is not equipped to answer these questions; ecological optics is, and some flavour of geometry will come in handy because geometry is just a way of talking about precisely the kinds of things described by ecological optics. It will remain, as ever, an empirical question as to which geometry is best suited, and when.

References
Fodor, J. A., & Pylyshyn, Z. W. (1981). How direct is visual perception?: Some reflections on Gibson’s “Ecological Approach”. Cognition, 9,139-196. DOI

Todd, JT, Oomes, AHJ, Koenderink, JJ, & Kappers, AML (2001). On the affine structure of perceptual space. Psychological Science, 12(3), 191-196. DOI

Turvey, M. T., Shaw, R. E., Reed, E. S., & Mace W. M. (1981). Ecological laws of perceiving and acting: In reply to Fodor and Pylyshyn (1981). Cognition, 9, 237-304. DOI

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