Depth (n.) needs no far travel; you can go down into it in your own back garden.
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Depth (n.) is claimed for theorems a ten-year-old could follow; how does a proposition come to have a deep content?
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Depth (n.) can be seen, and a flat retina is no argument against it; the problem is that we describe depth with a gesture, not a transparency.
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Depth (n.) seen in a picture on the wall is as ordinary as it seems — and so are many things.
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Depth (n.) is what we naturally see and draw; flat representation takes special training — hence the queerness of children's drawings.
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