The separation between sciences is crumbling. Nature doesn’t recognise disciplinary borders, and as we deepen our understanding, we see more of what these traditionally distinct branches of science have in common. There remain, however, curious hold-outs.

Physics deals with the basic properties of matter and energy and how they interact. Chemistry asks how atoms get together to form more complex molecules and what effect this has on the resulting substances. What both have in common is that they study inanimate matter.

Biology, on the other hand, studies living organisms. And here we encounter the central obstacle to seeing all of natural science as one big coherent whole. Inanimate matter seems to obey the laws of nature without exception and down to the last letter. Living things, by contrast, appear to have a will of their own. They are best understood — perhaps even best defined — by what might be called purposiveness. They try to do things, and while they cannot violate the laws of nature, they certainly can exploit them in order to realise their goals. You can’t say the same for inanimate matter.

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