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In Plato’s “Allegory of the Cave,” which addresses the origin of knowledge, his early insight into the constructive nature of perception offers illuminating metaphors for the process. The parable begins with the premise that a group of prisoners has never seen the outside world. Their experience is limited to shadows cast upon the wall of the cave by objects passing before a fire. The causes of those shadows—even the fact that they are shadows—is unknown to the prisoners. Nonetheless, over time, the shadows become imbued with meaning in the prisoners’ minds. Metaphorically, the shadows represent sensations, which are fleeting and incoherent. The assignment of meaning represents the construction of intelligible percepts. The prisoner turning the corner of the wall has been freed to witness the larger world of causes, which he reports back to those still imprisoned. In a novel metaphorical take on this ancient story, this return­ing prisoner represents the field of modern neuroscience, which sheds light on the relationship between our shadowy sensations and our rich perceptual experience of the world. (Plato’s Cave, 1604. Jan Pietersz Saenredam, after Cornelis Cornelisz van Haarlem. National Gallery, Washington D.C.)

I understood that the world was nothing: a mechanical chaos of casual, brute enmity on which we stupidly impose our hopes and fears. I understand that, finally and absolutely, I alone exist. All the rest, I saw, is merely what pushes me, or what I push against, blindly—as blindly as all that is not myself pushes back. I create the whole universe, blink by blink…. Nevertheless, something will come of all this.1


JOHN GARDNER’S HEARTRENDING TALE OF THE TORMENTED MONSTER Grendel’s perspective on life captures the fundamen­tal nature of perceptual experience: It is a construct that we alone impose. Or, as Grendel keenly observes, “The mountains are what I define them as.” Isolated and tortured by loneliness, Grendel sees the world as do the shackled prisoners in Plato’s Cave, where mere shad­ows are what is sensed, but those shadows are imbued with meaning, utility, agency, beauty, joy, and sadness, all through the constructive process of perception: “What I see, I inspire with usefulness … and all that I do not see is useless, void.”

Like the prisoner who escapes from Plato’s Cave to view a larger world of causes, or the all-knowing dragon who fills Grendel with ideas from another dimension—“But dragons, my boy, have a whole different kind of mind. … We see from the mountaintop: all time, all space”—modern neuroscience promises a mountaintop understand­ing of perceptual experience, an understanding not simply of the things we construct from our shadowy sensations, but how we do so, and for what purpose.

This section on perception offers that expansive mountaintop view. For each of the sensory modalities in turn, these chapters begin by examining environmental stimuli—light, sound, gravity, touch, and chemicals—that are the origins ...

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