living things where the need is to effect controlled collisions (e.g., soft collisions with little or no momentum exchange as in a bird landing on a branch, hard collisions with substantial momentum
|Time-to-contact,, is determined by the inverse of the relative rate of expansion of the optical flow field, A. (From Swenson, 1997a. Copyright 1997 JAI Press. Used by permission).|
exchange as when a predator attacks a prey,
and collision avoidance where the ends of an intentional agent
require that it not collide with particular things). In particular,
as shown in Figure 9, time-to-contact ( ) is determined by the
inverse of the relative rate of expansion of the optical flow
field, and the information about whether a collision will be hard
or soft is given by the time derivative or rate of change of the
relative rate of expansion ( ) (Lee, 1980; Kim et al. 1993). In
the case of a bird landing on a branch, and requiring a soft collision,
for example, the rate of change must be . This example shows how
a single macroscopic variable nomologically carried in the optic
flow, can precisely determine the intentional dynamics of living
things, in this case when a particular bird, for example, must
open its wings to decelerate now so that it does not, in effect,
crash into a branch later.
This and other related examples expose the fact that not only are the shapes and forms things assume nomologically determined by laws of form (e.g., that there is, within tolerance, a requisite ratio between flight muscle weight and body weight, or between wing span and body weight, or brain weight and body weight [e.g., Alexander, 1971]), but that semantic content carried in macroscopic flow variables nomologically determines the behavior of things, within tolerance, towards their intentional ends. The major contribution of this deceivingly simple insight is that it gets meaning outside of heads or disembodied mental states and into the context of intentional dynamics and its entailments, or into the world itself. It shows precisely how the ecological or self-other relations that characterizes intentional dynamics can be nomologically instantiated are maintained, and it is of particular interest, although Gibson did not note it, that the invariant macroscopic properties to which he pointed are precisely a consequence of first law symmetry, or the time-symmetry, of the space-time continuum itself (Swenson, 1997a, in press a, b).
Finally, in conclusion, older views built on Cartesian assumptions typically find themselves with the 'Problem of Parmenides', with a postulated world or ontology that cannot account for or even accommodate the epistemic act of the subject doing the postulating, and where ecological relations are ex hypothesi inimical. Abandoning these untenable Cartesian assumptions of incommensurability (e.g., between knower and known, subject and object, or the "mental" and "physical), it is now possible to build a principled foundation for a theory which unites these two otherwise incommensurable parts, and where what is given a priori as the intentionality of the epistemic act, and a posteriori as the epistemic development of space-time, rather than being 'outside tolerance' of natural law or universal principles is seen instead to be a direct manifestation of them.