Steven Pinker -- How the Mind Works - Hampshire High Italian ...

278 | HOW THE MIND WORKSexplanation, because a geometer would say they are no different from upand down or front and back.The explanation is that mirror-image confusions come naturally to abilaterally symmetrical animal. A perfectly symmetrical creature is logicallyincapable of telling left from right (unless it could react to thedecay of cobalt 60!). Natural selection had little incentive to build animalsasymmetrically so that they could mentally represent shapes differentlyfrom their reflections. Actually, this puts it backwards: naturalselection had every incentive to build animals symmetrically so that theywould not represent shapes differently from their reflections. In theintermediate-sized world in which animals spend their days (bigger thansubatomic particles and organic molecules, smaller than a weatherfront), left and right make no difference. Objects from dandelions tomountains have tops that differ conspicuously from their bottoms, andmost things that move have fronts that differ conspicuously from theirbehinds. But no natural object has a left side that differs nonrandomlyfrom its right, making its mirror-image version behave differently. If apredator comes from the right, next time it might come from the left.Anything learned from the first encounter should generalize to the mirror-imageversion. Another way of putting it is that if you took a photographicslide of any natural scene, it would be obvious if someone hadturned it upside down, but you wouldn't notice if someone had flipped itleft-to-right, unless the scene contained a human-made object like a caror writing.And that brings us back to letters and mental rotation. In a fewhuman activities, like driving and writing, left and right do make a difference,and we learn to tell them apart. How? The human brain andbody are slightly asymmetrical. One hand is dominant, owing to theasymmetry of the brain, and we can feel the difference. (Older dictionariesused to define "right" as the side of the body with the strongerhand, based on the assumption that people are righties. More recentdictionaries, perhaps out of respect for an oppressed minority, use a differentasymmetrical object, the earth, and define "right" as east whenyou are facing north.) The usual way that people tell an object from itsmirror image is by turning it so it faces up and forward and looking atwhich side of their body—the side with the dominant hand or the sidewith the nondominant hand—the distinctive part is pointing to. Theperson's body is used as the asymmetrical frame of reference that makesthe distinction between a shape and its mirror image logically possible.