"Nature without learning is like a blind man; learning without Nature, like a maimed one; practice without both, incomplete. As in agriculture a good soil is first sought for, then a skilful husbandman, and then good seed; in the same way nature corresponds to the soil, the teacher to the husbandman, precepts and instruction to the seed."

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One can see from space how the human race has changed the Earth. Nearly all of the available land has been cleared of forest and is now used for agriculture or urban development. The polar icecaps are shrinking and the desert areas are increasing. At night, the Earth is no longer dark, but large areas are lit up. All of this is evidence that human exploitation of the planet is reaching a critical limit. But human demands and expectations are ever-increasing. We cannot continue to pollute the atmosphere, poison the ocean and exhaust the land. There isn't any more available.

— Stephen Hawking