The philosopher, George Berkeley, famously asserted that we cannot see distance. Our eyes can only see in 2-dimensions. However, we manage to navigate in a 3-dimensional world pretty well. 3-dimensional seeing then, is a learnt skill. This conclusion has radical implications for the possible development of perception to new and novel places, writes Kenneth L. Pearce.

John Locke credits the Irish philosopher and vision scientist William Molyneux (1656–1698) with posing the following ‘problem’:

Suppose a man born blind, and now adult, and taught by his touch to distinguish between a cube and a sphere of the same metal, and nighly [nearly] of the same bigness, so as to tell, when he felt one and the other, which is the cube, which the sphere. Suppose then the cube and sphere placed on a table, and the blind man be made to see: Quaere [Question], “whether by his sight, before he touched them, he could now distinguish and tell, which is the globe, which the cube?” (Essay concerning Human Understanding, §2.9.8)

More than 300 years later, Molyneux’s Problem (as it has come to be known) has not been definitively answered and continues to inspire research in neuroscience, developmental psychology, and philosophy. The fundamental question it raises is: what is the role of learning or experience in perception? Might people with different experience literally see the world differently?

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In other words, seeing tangible, three-dimensional shapes is something we learn.

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One of the central theses of Locke’s Essay is that all of our ideas are derived from experience. Much of this experience comes through our five senses. Some of our ideas are derived from one sense only. For instance, you can’t taste or hear the color red, you can only see it. But other ideas seem to be common to more than one sense. For instance, if your dog is barking joyously as she returns from rolling in some particularly delightful filth, you can tell her distance from you by hearing, sight, smell, and (once she’s close enough) touch. The notion of distance, then, would seem to be delivered by at least four of the five senses. Similarly, we learn the shapes of things by (at least) both sight and touch. Thus, Locke holds that “we can receive and convey into our minds the ideas of the extension, figure, motion, and rest of bodies, both by seeing and feeling” (Essay, ch. 2.5).

If this were true, however, we would expect the answer to Molyneux’s Problem to be ‘yes’. If one and the same idea of cube is delivered to us by both sight and touch, then the formerly blind man ought to be able to recognize the idea he receives by sight as the same familiar cube he always knew by touch. Yet both Locke and Molyneux hypothesise that he will not be able to do this.

In his first major philosophical work, An Essay toward a New Theory of Vision (1709), George Berkeley argued that Locke and Molyneux had failed to recognize the radical consequences of their negative answer to Molyneux’s Problem. According to Berkeley, if it is really true that the formerly blind man would not immediately recognize the cube by sight, this implies that the idea we have when we see the cube is entirely different from the idea we have when we touch it, and any connection between the two must rely entirely on an experienced correlation. In other words, seeing tangible, three-dimensional shapes is something we learn.

Locke had already suggested that experience might affect how we see. In his discussion of Molyneux’s Problem he gives a vivid example: “a man who reads or hears with attention and understanding, takes little notice of the characters, or sounds, but of the ideas that are excited in him by them” (Essay, §2.9.9). Once you have learned a language, you can’t return to hearing it as mere sound. Once you’ve learned to read, you can’t return to viewing the marks on the page as mere squiggles. Instead, the meaningfulness of the words is part of what you experience.

The Scottish philosopher Thomas Reid (1710–1796) dubbed this phenomenon ‘acquired perception’. Examples abound. Reid mentions the case of a butcher who can see the weight of a sheep—just by looking, with his eyes. The present day philosopher Susanna Siegel has discussed at some length the ways in which learning to distinguish pines from firs might alter your perception of a forest.

Reflection on Molyneux’s Problem, and the question of whether the same idea can be perceived by two senses, led Berkeley to a much more radical position than is suggested by these examples. According to Berkeley, what is initially delivered to us by vision is only a field of light and color, corresponding to the two-dimensional projection on the retina of the eye. Just as we must learn to see or hear the meaning of words, so also we must learn to see three-dimensional objects at a distance from us in space. Indeed, according to Berkeley, vision itself is a language, and we acquire it in infancy just as we might acquire a language such as English, Korean, or Swahili.

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The fact that our way of seeing the illusion is influenced by culture shows that it is a product of experience, and not biologically hard-wired.

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Berkeley’s tight connection between language learning and visual processing is generally not endorsed in later scientific literature. However, Berkeley’s general idea that we learn by experience to understand visual stimulus has been enormously influential in neuroscience and developmental psychology. In early infancy, children spend a great deal of time and energy reaching, grasping, and moving. They watch with their eyes the movement of their own limbs and other objects in their environment. While recent studies suggest that some rudimentary ability to connect vision and touch is present from birth, the ability to fully integrate these two sources of information develops much later, not reaching adult levels until 8 to 10 years of age. In an article summarizing the current state of experimental research on these matters, David Burr and Monica Gori write:

In his 300-year-old “Essay towards a new theory of vision,” Bishop George Berkeley (1709) correctly observed that vision has no direct access to attributes such as distance, solidarity [sic], or “bigness.” These can be acquired visually only after they have been associated with touch (proposition 45): in other words, “touch educates vision,” perhaps better expressed as “touch calibrates vision.” Calibration is probably necessary at all ages, but during the early years of life, when children are effectively “learning to see,” calibration may be expected to be more important.

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The butcher really can see the weight of the sheep, in just the same sense that she can see its distance.

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What are the consequences of these observations for our knowledge of the world? One might see cause for pessimism here, since we know that learning from experience can go wrong. As a result, having the wrong course of childhood experience might lead us to perceive certain things wrongly. Interestingly, there may be real world examples of this. For instance, research has shown significant cross-cultural variation in responses to the Müller-Lyer illusion—in particular, Europeans find the illusion much more powerful than members of traditional African cultures. The cause of the illusion, and the reason for this cross-cultural variation, is not well understood. Some have hypothesised that susceptibility to the illusion is a result of living in ‘carpentered’ environments with lots of right angles. Others have hypothesised that it’s due to spending a lot of time looking at two-dimensional images of three-dimensional objects. Whatever the case, the fact that our way of seeing the illusion is influenced by culture shows that it is a product of experience, and not biologically hard-wired. Somehow I’ve learned to perceive these lines incorrectly.

Figure 1: The Müller-Lyer illusion. All of the horizontal lines are the same length.

On the other hand, there is an optimistic conclusion to be drawn. Humans, on this view, have the potential to see the world in ways that go far beyond the innate endowment that we have from our sense organs. The butcher really can see the weight of the sheep, in just the same sense that she can see its distance.

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Precisely because acquired perception is shaped by experience, a variety of incompatible ways of perceiving the world are possible.

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If acquired perception can confer knowledge, then skillful perception, developed through a proper course of education, could in principle confer effortless knowledge of just about anything. Berkeley, indeed, makes a rather surprising claim to this effect. According to him, if we learn to perceive the world rightly, we will be able to “see God…[in] everything we see, hear, feel, or anywise perceive by sense’’ (A Treatise concerning the Principles of Human Knowledge, §148). In Berkeley’s view, religious experience can be, and ought to be, pervasive. Further, religious experience functions just the same as mundane sorts of perception, such as seeing distance. And, just like distance vision, religious experience is learned.

If the theory of acquired perception is correct then we can learn to perceive the world as the immediate product of a providential God, a ‘fortuitous concourse of atoms’, a quantum wavefunction, or the veil of Maya. The possibilities are endless. But how can we know we’ve learned to perceive the world correctly?

If we believe that we can know a three-dimensional world by sight, then we must accept that acquired perception can, when things go right, confer knowledge. Yet, precisely because acquired perception is shaped by experience, a variety of incompatible ways of perceiving the world are possible. There are numerous underexplored puzzles for the theory of knowledge here. Perhaps the most important conclusion to draw is that perceiving the world correctly is a learned skill. This skill is of the utmost importance if we are to succeed in knowing the world around us, and yet it is very difficult to be sure we’ve acquired it.