This research seeks general psychological laws that can be understood as adaptations to universal characteristics of the world in which we have evolved. Universal characteristics of our world include, for example, the following: Space is three- dimensional and Euclidean. Objects have inherent properties that persist over time despite variations in their momentary positions, conditions of illumination, etc. Objects belonging to a significant natural category, although differing somewhat among themselves, tend to be similar enough to each other to form a compact region (having some location, size, and shape) in the abstract "space" of possible objects. Examples of psychological principles that can be shown to be adaptive in a world with such characteristics include the following three: First, certain particular motions of an object in space are psychologically favored in the sense that those motions (a) are judged simplest or most natural when physically presented, (b) are most likely to be experienced in the visual illusion of "apparent" motion, which arises when one object is suddenly caused to disappear and immediately to reappear in a different position, or (c) are most likely to be imagined when two similar objects in different positions must be compared mentally with respect to shape. Second, colors have three degrees of freedom in the sense that any given color can be matched by adjusting just three knobs on a suitable color-mixing apparatus. Third, the probability that a response learned to one stimulus will generalize to another stimulus decays uniformly according to an exponential function of the (properly defined) degree of difference between those two stimuli, while the time required to make a discriminative response indicating that the two stimuli are different decays according to a different, reciprocal (or hyperbolic) function of that difference. Such laws cannot find their ultimate explanation in terms of neurophysiological structures, which themselves must be explained; they can however be understood as realizing an optimum strategy in a world in which objects having biologically significant consequences belong to natural kinds. Such results suggest that the mind operates in accordance with invariant psychological laws, not unlike the invariant laws of physics, provided that the psychological laws are formulated in terms of the unique "psychological space" yielding invariance and not in terms of the "physical space" defined by physical measurement. The psychological space is presumed to be related to the physical space by a psychophysical transformation that has been shaped by eons of natural selection.
