Affordances are difficult entities to wrap your head around. We talk about them imprecisely, they seem like odd, ghostly entities that couldn't possibly exist, and even when we get precise about them, we end up with two different accounts of the kind of thing they are.
Gibson did not have a logical framework with which to express what he mean by affordances, although his verbal formulation is quite clear:
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)
There have been two attempts at formalising this definition, with two incompatible results: for Turvey, an affordance is a dispositional property of the environment, like solubility, which is complemented by an effectivity of an organism. For Stoffregan and Chemero, affordances are relations between the organism and environment. These are not the same and the difference matters: so which is it?
For a long time I was in the relations camp. Affordances 'point both ways', in Gibson's phrase; they are only defined with reference to an organism, and hence must be relational. This doesn't affect the essential realist stance of the ecological approach, because we are 'radical' - relations are just as real as properties, and can hence serve as objects of perception. Defining affordances as relations also allows you to solve a troubling aspect of Turvey's dispositions, namely the compulsory actualising or manifesting of the disposition in the presence of its complement. Clearly we do not attempt to realise all the behaviours afforded to us; behaviour is selective.
I was all about this when I read Chemero's recent book, but got schooled by my PhD advisor - no, he said, affordances are dispositional properties, not relations. Geoff is generally right about things like this, so while I couldn't see how it could work I kept mulling it over. Reading Heft (2001), Chapter 3, I had an intuition about how this could work, and I've now officially re-converted - affordances must be dispositional properties of the environment, so that they can persist when we aren't around and so we can perceive how to effect a currently impossible behaviour.
Affordances must persist when there is no-one around to effect them
Defining affordances as dispositions solves at least one important problem raised by defining them as relations: dispositions exist even when the complementary property is not present, and hence affordances, if they are dispositions, exist as potential in the environment before any organism is present to effect it. Stoffregan and Chemero both dodge this bullet in a deeply unsatisfying way - they simply declare that affordance relations persist, without any stated reason.
Why does this matter? Surely it makes no sense to say a surface affords locomotion if there is nothing around that needs to locomote. However, no one wants to say this, not even Chemero or Stoffregan; this is why they need the additional claim that the affordance relation persists. The reason for this is simple: the goal of ecological psychology is to create a naturalised theory of human perception and action. To naturalise a theory is to make it consistent with facts of biology and evolution, and this is a vital part of any theory about a biological organism. Modern cognitive psychology runs circles round itself whenever it tries to think about this problem, because the information processing metaphor is simply not up to generating a naturalised account of anything (as evidenced, I think, by the fact that biologists never ever invoke the metaphor). Ecological psychology aims to explain perception and action in a manner that is fully consistent with theories of biology; accounts of our behaviour therefore require an account of our niche, and hence affordances, which are what we perceive in order to control our behaviour, must be persistent features of our experience so this niche can be sensibly defined. Relational accounts include the requisite stable properties of the environment; they just don't call those bits the affordance. Chemero in particular thinks that only this relational account achieves the required naturalisation.
Affordances for behaviour we cannot currently effect
But we still have a problem - if we directly perceive affordances in order to control our behaviour, and affordances are the relation between our abilities and properties of the world, then we can only directly perceive opportunities for behaviour that we can do, right now. This has one glaring problem: we cannot directly perceive an opportunity for behaviour that we cannot currently effect, nor what is required to effect that behaviour. However, we clearly do this all the time: even a young child knows that if they cannot reach the cookie jar on the bench, their options include getting a stool they can climb that will bring the jar into reach, or finding an adult who can reach the jar.
To try and formalise this a little: we can perceive the possibility of actions we can't effect, and we can perceive the possibility of an action for another organism. Possibilities for action should be affordances, but if affordances are the relationship between our abilities and the environment, these other possibilities aren't available to us and hence aren't affordances. But these possibilities are available to us, and crucially so: if we could never know that 'we could reach that cookie jar if only I was slightly taller' we could never adapt our behaviour to achieve our goals because we wouldn't know where to begin (or where our adaptation was heading) - we would be stuck in the moment and our behaviour could not flexibly change over time.
website of ecological psychologist Michael J Richardson); watch especially as the stick figure seamlessly solves the problem of the step that is initially too high in multiple ways.
I'm still thinking about this, and trying to get the formalism right; the problem of compulsory effecting of a disposition also needs to be addressed. But the relational account simply does not stand up to scrutiny and does not allow the kinds of behaviour we must be able to account for. Our environmental niche contains affordances, some presented by nature and some by artificial alterations. Biological evolution is the ongoing process of evolving effectivities that match our niche; invention and artefact design is the ongoing cultural process of extending our range of effectivities to cope with novel affordances as we expand our range of niches to include cities, space, and all the crazy places people live on Earth. These affordances that define the niche must come prior to the organism who can then evolve into that niche. The net result is precisely as Gibson described - affordances point both ways, but only if they are dispositions. A given niche implies an organism with specific abilities as much as an organism with specific abilities implies a particular niche. No organism could ever behave functionally if its possibilities for behaviour swung in and out of existence as it moved around the world, nor could it ever evolve if those possibilities were limited to what it could currently achieve.