Gibson hypothesized that agents evolved to "directly" access relevant information about themselves and the environment from the invariant structures in the array, without the need for high level cognitive computations. In other words, in the aforementioned case of objects appearing to grow or shrink, no cognitive processes mediate the observer sensing the apparent growth in size of the object and the observer perceiving that she has now moved toward the object (or the object has moved toward her). These invariant properties are linked with Gibson's idea of affordances. According to Gibson, an affordance is a property of the environment, much like color and size are. For an animal with the appropriate physiological equipment, a tree affords the ability to climb up it, or the ground the ability to walk upon it. Therefore, he claimed, affordances are also specified in the ambient optic array. This means that not only can an agent directly perceive that there is a horizontal surface or that a tree is a tree, but that a horizontal surface is "walk-on-able" or that a tree is "climb-up-able". In fact, the agent's perceptual system is so attuned to the invariant information, Gibson argues, that the agent need not consult any of its prior experiences in order to interact with the environment. This implies that agents pick-up meaning and value directly from the environment, rather than project it onto the world. Many critics have rejected at least some of Gibson's claims. Psychologist Richard Gregory asserted that Gibson's bottom-up approach to perception is incomplete. He argued that visual illusions like the Necker cube are the result of the brain's indecision between two equally plausible hypotheses about the cube's orientation. The cube appears to "flip" between these two orientations even though the sensory information remains static. Therefore, Gregory reasoned that top-down processes must mediate perception. In response, Gibson argued that illusions like the Necker cube are the result of artifice and would not be encountered by agents in realistic perceptual situations, and therefore are irrelevant. However, the waterfall illusion is an example of a naturally occurring illusion and cannot be accounted for by Gibson's theory. Nevertheless, these two approaches can be reconciled. For example, Ulric Neisser developed the perceptual cycle, which involves top-down and bottom-up perceptual processes interacting and informing each other. The processes are causally linked, but of equal importance. Furthermore, David Marr claimed that Gibson had profoundly underestimated the intricacy of visual information processing. While useful information may exist directly in the ambient optic array, Gibson does not elaborate on the mechanisms of the direct pick-up of this information.