Affordances, Part 2: Affordances are relations between organism and environment

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.