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So the atmosphere is a part, a most necessary one, of the world. This it is that joins heaven and earth, separating highest and lowest in such a way as yet to unite them. It separates by coming in between, it unites by rendering possible communication between the two. It transmits to the higher regions what it receives from the earth; and again, it transfuses terrestrial objects with the influences of the heavenly bodies. I call it a part of the world in the same sense as animals and trees are parts. The whole class of animals and trees forms part of the universe, since it has to be taken in to make up the whole, and without it the universe is not complete. A single animal or tree is a quasi-part: though it is lost, that from which it is lost is still entire. Now the atmosphere, as I have been saying, adheres both to sky and earth. In both it is inborn. Whatever is an inborn part of anything else possesses unity, for without unity nothing can be born.