 There is a tradition in philosophy and psychology that denies, on phenomenal grounds, the distinction between impoverished, typically two-dimensional sensory contents or states and the perception of objects in three dimensions. One can see William James’s affirmation that sensations are intrinsically world-directed (1890, 2: 1–3) and exhibit three-dimensional volume (1890, 2: 135–136) as objecting in advance to the Classical Notion of sense data. James denies  item 5,  that sense data are neutral about a world beyond them, in favor of a position that both sensation and perception give us “an immediately present outward reality” (1890, 2: 2). The Gestalt psychologists (Koffka 1935: chs. 1–3; Köhler 1929 [1947: chs. 1–3]) also adopted a position in which we are perceptually given a world, not an impoverished content or object distinct from the world. They made perception of the valence or functional values of the world into an immediately given aspect of perceptual content. They also denied that two-dimensional sensations or sense data are the core from which perceptions are developed: the world is given in  three-dimensions.[21]  The Gestalt theorists drew attention to the phenomena of size and shape constancy. If two objects of the same size are presented at five and ten feet, the retinal image of the farther object will be half as large (in linear height) as the image of the nearby object; yet observers report them to be phenomenally of the same size (or nearly so). Similarly, a circle seen at a (moderate) slant projects an ellipse on the retina but appears circular. These theorists contended that significant phenomenal differences in size between the two objects, and the appearance of an elliptical shape, are produced only under special laboratory conditions or through trained acts of attention, as might be found in those who are taught to draw (Koffka 1935: 222–223; Köhler 1929 [1947: 71–74]). Subsequently, the perceptual psychologist James J. Gibson (1950) agreed with the Gestalt theorists in arguing that the two-dimensional experiences that are like sense data arise from acts of attention that artificially alter our phenomenology away from the three-dimensional visual world that we normally perceive. Accordingly, the two-dimensional visual field is not an immediate object of perception or primitive constituent of perception that is uncovered by attention. Rather, it is constructed through a special effort of attention. Gibson held that, in everyday perception of a circle at a slant (say, your neighbor’s plate at a dinner party), the object looks like a circle at a slant and so looks circular, not elliptical (1950: chs. 1, 9). In subsequent writings, Gibson (1966, 1979) developed the notion that sensory systems are attuned for presenting to perceivers a navigable environment and its affordances for meeting the perceiver’s needs and goals.