Ecological psychology views goal-directed behavior as a process by which a cognitive operation, an intention, selects an affordance property of the environment to be a goal, and then assembles and sustains an effectivity, the means for attaining that goal. Under this formulation, goal-directed behaviors require the perceptual scaling of affordances to effectivities under intentions. In support of this theory, five experiments were conducted to study the perception of the affordance of passability for walkers and wheelchair-users through doorways.^ First it is demonstrated that for perception of passability, geometrical scalars are not simply in reference to body size, rather they are functional in nature. Body dimensions which generally do not determine tolerance of fit are less tuned for. Tool-use is discussed to suggest that geometrical scaling can be by a non-body scalar. Second tuning is examined directly. Learning to use a tool (wheelchair), as the change of effectivities, allowed us to observe that tuning requires a complete perception-action cycle in the relevant environment. Third, a study of perceptual scaling for passability of doorways made of different material reveal that meaning is a scalar that can be mapped geometrically in concert with kinematic, and kinetic parameters. Finally, visual judgment and performance are compared directly. It is shown that participants\u27 judgments of what they can do (perceptual scaling) is mapped 1:1 with what they can actually do. That mapping of affordances to effectivities is extraordinarily constrained by the manner selected to attain that goal, that is, by an intention. In this process I examine kinematic parameters, which are manner parameters, as scalars. Conclusions are drawn from the above for psychology and for engineering.