The idea that affordances are dispositional properties of the environment is not immediately obvious, given the way everyone talks about them. This was certainly the source of my confusion - affordances are always described as animal-relative properties of the environment, and in the classic Warren stair-climbing affordance studies Bill Warren pioneered the use of pi numbers to describe affordances. Pi numbers are a handy trick from engineering, where you characterise a relation using a ratio. This has the advantage of a) eliminating the units so you can then rescale the number in any system you like, and b) highlighting that the relation between two things remains common even when the specific details change. Warren found that judgements of climbability varied not with leg length or riser height but with the ratio. Affordances are therefore almost always discussed this way - as being about the relation between an organism and its environment. Ecological psychology eschews internal representation and computation, so this relation must be directly perceived, i.e. they must be affordances.
Several people have been advocating such a view for several years. Thomas Stoffregan has talked in at least two papers about affordances as properties of the animal-organism environment (Stoffregan 2000, 2003). In terms of the logical structure in my last post, the affordance is r, not p. Anthony Chemero has advocated specifically for affordances to be relations (Chemero, 2003) and more recently that affordances aren't just defined across the animal-environment system, but that they causally interact with abilities (Turvey's effectivities, although without the implication that they complement a disposition) over time (Affordances 2.0; Chemero, 2009).
Dispositions are always actualised in the presence of their complement
There is essentially one main objection to the affordances-as-dispositions view. Turvey described three characteristics of dispositions; the third is
6.3. Dispositionals never fail to be actualized when conjoined with suitable circumstances. Dispositions and suitable circumstances equals actuality. (Turvey, 1992, p. 178)
In any given situation, an organism is part of a huge web of possible behaviours, but does not try to effect them all; indeed, none of the possible actions seem compulsory (this is surely implicit in the notion of possible). Turvey's 'solution' is to acknowledge the problem and hand off responsibility of selection to j. a 'juxtaposition function' that acts so as to select the one affordance that will be effected. For Stoffregan, the fact that there are unrealized affordances means they simply cannot be dispositions, by the definition in 6.3 above; they are instead a higher order property of the animal-environment system that do not belong to either individually. Behaviour (the selection and effecting of an affordance) occurs when intention and an affordance intersect.
Chemero has a similar problem and solution; but affordances aren't properties at all, not even of the higher-order system: they are explicitly relations, which any ecological psychologist will happily accept can be real and perceived (following James, Holt and Gibson in their realist stance). In his book, Chemero (2009) expands on this further (into what he calls Affordances 2.0) in order to make affordances and abilities co-evolve over time.
Do relations continue to exist without the propertied things that make the relation?
Any relational account runs immediately into a problem: do stairs afford climbing if there's no-one around to climb them? It's actually quite important that they do, but I have yet to see a good answer to this question. Chemero (2009) simply declares that they do (p.149-150), as does Stoffregan (2000; p. 7), although Stoffregan (2003, p 125) seems to be happy with the idea that affordances can persist even when not effected by an organism without any discussion of what happens when the organism leaves the room (so to speak).
So that's the basic state of affairs; affordances certainly involve both the organism and environment in a mutual relationship, but Gibson did not have a logical framework in mind when he defined affordances in 1979:
An affordance is neither an objective property nor a subjective property; or it is both if you like. An affordance cuts across the dichotomy of subjective-objective and helps us to understand its inadequacy. It is equally a fact of the environment and a fact of behaviour. It is both physical and psychical, yet neither. An affordance points both ways, to the environment and the observer. (Gibson, 1979, p. 129)
The dispositional and relational views are both attempts to place this clear (if slightly mysterious sounding) definition within a formal logical framework. Neither is without problems. Dispositions are actualised in the presence of their complement, so how do we explain selectivity of behaviour? Relations are real, but do they even exist when the propertied things that define the relation do not? My next post on this will be some first thoughts on how and why I've changed my mind from the relational back to the dispositional view, although I freely admit my thinking is in flux on this right now.
Chemero, A. (2003). An Outline of a Theory of Affordances Ecological Psychology, 15 (2), 181-195 DOI: 10.1207/S15326969ECO1502_5
Stoffregen, T. (2000). Affordances and Events Ecological Psychology, 12 (1), 1-28 DOI: 10.1207/S15326969ECO1201_1
Stoffregen, T. (2003). Affordances as Properties of the Animal-Environment System Ecological Psychology, 15 (2), 115-134 DOI: 10.1207/S15326969ECO1502_2
Turvey, MT (1992). Affordances and prospective control: An outline of the ontology. Ecological Psychology, 4(3), 173-187. DOI: 10.1207/s15326969eco0403_3