We see in two dimensions. It sounds strange, but have a look around you. What you see with your two eyes is that some things are smaller and some are larger, but the only data that comes to us through our eyes is height and width. Depth is a magical addition we give the world through years of experience in knowing whether a small thing is far away, or just small. If you throw a baseball to a friend standing 10 metres away, the only information each eye gives you is that the ball is shrinking until it arrives at your friend, who’s smaller than she’d be if she were closer to you, and larger than she’d be if she were farther away.

That’s the one-eyed picture of the world. But once we add a second eye which gets a slightly different image because of its gap with the first, depth just seems to appear. The most miraculous thing about the optical illusion that we call our perception of the world is that we think nothing of it. Yet the ability to assemble two slightly different images of the world and – fairly accurately – interpret a third dimension gives a person a huge advantage over cyclopses and pirates who can’t be sure whether pterodactyls are swooping down at them, or just growing really quickly.

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