|Alice had a few problems with doors too|
Case in point: in 1987 Bill Warren was investigating the perception of affordances for passing through an aperture, such as a door. Apertures can vary in width, but the question facing the observer isn't 'how wide is it?' but 'can I fit through it?'. Bill was describing affordances using pi numbers, ratios of widths in the world divided by widths of some relevant body scale. An aperture wider than your shoulders, say, would yield a ratio larger than 1. For a given person's shoulder width, different apertures would produce different ratios. Across people, apertures of different sizes will produce the same ratio as shoulder width changes. It's a way of factoring out the effect of the size of the observer, and it's based on the idea that we percieve the world in terms of our own ability to act on it.