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“KNOW thyself.” The first of three maxims said to have been inscribed in the forecourt of the Temple of Apollo in Delphi sounds grand. What it actually means has been a matter of debate for millennia, and when it comes to knowing ourselves, modern science has made things deliciously more complex, too.

How the physical substance of our bodies creates our sense of being a consistent entity, and what it means to have that sensation, is a long-standing puzzle. Debates about this relationship between matter and mind were meat and drink to the Ancient Greek philosophers, but …