An Essay Towards a New Theory of Vision

How can the eye tell how far away a thing is, when depth arrives at it as a single point with no length? Berkeley throws out the hidden geometry the optics of his day relied on.

Distance runs straight out from the eye, so it meets the retina as a single point that carries no clue to its length. The standard optics of the day, inherited through Kepler, tried to supply the missing information with unnoticed geometry — angles the mind was supposed to compute without knowing it. Berkeley, twenty-four and fresh from Trinity College, Dublin, threw the geometry out. We do not calculate distance, size and situation; the eye has no access to such angles. His problem is genuine and precise, and it forces a completely different account of how the given point of light becomes a seen world with depth.

• vision
• perception
• optics
• geometry
• distance
• magnitude
• mathematics
• distance perception

If the eye cannot compute depth, how do we learn to see it? Berkeley's answer is habit: the eye learns to read what it sees as a sign of what the hand would feel.

We judge distance, size and situation, Berkeley argues, not by mathematics but by habit. Through long correlation, the eye learns to associate what it sees with what the hand would touch and the body would do to reach it. Sight and touch are thus two languages, and we translate one into the other so fast that the translation feels like seeing itself. Visual cues become signs of tactile realities, connected by experience rather than by nature or geometry. This makes spatial perception something learned and read, like a language acquired in infancy, rather than something given whole in the image.

• perception
• touch
• language
• experience
• learning
• philosophy of perception
• visual perception
• philosophy of vision

Do the eye and the hand grasp the same shape? Berkeley says no — and answers the old puzzle of the newly sighted with a firm denial that unsettles common sense.

The deep claim behind the essay is that sight and touch share no idea in common: the visual shape of a thing differs in kind, not merely in degree, from the shape the hand would feel. So Berkeley takes up the Molyneux problem — whether a man born blind and suddenly given sight would recognize a cube by looking — and answers no. Having never learned to link the two languages, the newly sighted person would see only meaningless color and light, unable to match it to anything felt. The heterogeneity of the senses is what makes the whole theory of vision necessary rather than optional.

• experience
• learning
• touch
• perception
• apparent magnitude
• sense perception
• signs
• cross-modal perception