Why are we the size we are, instead of some very different size? Simple physical scaling laws and three ’’requirements’’ dictate that our size be of order (h/2/mee2)(e2/Gmp2)1/4. They also ’’predict’’ the mass and radius of the Earth. The three requirements are: (i) We are made of complicated molecules; (ii) we breathe an evolved planetary atmosphere; (iii) we are about as big as we can be without breaking.

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