It is because the affordances of things for an observer are specified in stimulus information. They seem to be perceived directly because they are perceived directly.…
The central question for the theory of affordances is not whether they exist and are real but whether information is available in ambient light for perceiving them. The skeptic may now be convinced that there is information in light for some properties of a surface but not for such a property as being good to eat. The taste of a thing, he will say, is not specified in light; you can see its form and color and texture but not its palatability; you have to taste it for that. (Gibson, 1979, p. 140)
In comments on a post from weeks ago, Gennady Erlikmann draws our attention to some important texts from the latter portions of Chapter 8 of Gibson, 1979. Now, it seems to me that the passages from p. 140 cited above champion the view that affordances structure light in such a way as to enable the affordances to be perceived. And he does not there back off the view. But, then on p. 142, he apparently does back off of this apparently admitting that sometimes affordances do not structure light in such a way as to enable them to be perceived. (That's the italized part in paragraph 3 from p. 142.) In the final paragraph, the two italicized sections suggest that the glass in the two cases did not structure light in such a way as to specify an affordance of, say, "stand-on-ability" or "walk-through-ability". Instead, the glass apparently specified something else, namely, air. Kind of grudging admissions.MISINFORMATION FOR AFFORDANCESIf there is information in the ambient light for the affordances of things, can there also be misinformation? According to the theory being developed, if information is picked up perception results; if misinformation is picked up misperception results.The brink of a cliff affords falling off; it is in fact dangerous and it looks dangerous to us. It seems to look dangerous to many other terrestrial animals besides ourselves including infant animals. Experimental studies have been made of this fact. If a sturdy sheet of plate glass is extended out over the edge it no longer affords falling and in fact is not dangerous, but it may still look dangerous. The optical information to specify depth-downward-at-an-edge is still present in the ambient light; for this reason the device was called a visual cliff by E. J. Gibson and R. D. Walk (1960). Haptic information was available to specify an adequate surface of support, but this was contradictory to the optical information. When human infants at the crawling stage of locomotion were tested with this apparatus, many of them would pat the glass with their hands but would not venture out on the surface. The babies misperceived the affordance of a transparent surface for support, and this result is not surprising.Similarly, an adult can misperceive the affordance of a sheet of glass by mistaking a closed glass door for an open doorway and attempting to walk through it. He then crashes into the barrier and is injured. The affordance of collision was not specified by the outflow of optical texture in the array, or it was insufficiently specified. He mistook glass for air. The occluding edges of the doorway were specified and the empty visual solid angle opened up symmetrically in the normal manner as he approached, so his behavior was properly controlled, but the imminence of collision was not noticed. A little dirt on the surface, or highlights, would have saved him.These two cases are instructive. In the first a surface of support was mistaken for air because the optic array specified air. In the second case a barrier was mistaken for air for the same reason. Air downward affords falling and is dangerous. Air forward affords passage and is safe. The mistaken perceptions led to inappropriate actions. (Gibson, 1979, p. 142)
Nevertheless, however true all this may be, the basic affordances of the environment are perceivable and are usually perceivable directly, without an excessive amount of learning. The basic properties of the environment that make an affordance are specified in the structure of ambient light, and hence the affordance itself is specified in ambient light. Moreover, an invariant variable that is commensurate with the body of the observer himself is more easily picked up than one not commensurate with his body. (Gibson, 1979, p. 143).In the final paragraph here, Gibson suggests that, while maaaybe not all affordances are perceived, at least all the basic affordances are perceived.
Now, I've been trying to give examples showing that this kind of analysis will not work. The box and the exploding box are physically the same on the outside, so structure light in the same way. There is no room to say that one of these structurings is information, where the other is misinformation. As light structure, they are the same. At most, calling one "information" and the other "misinformation" is just to rename the one light structure as coming from the normal box, where the other light structure as coming from the exploding box.