Nietzsche saw dreams as an evolutionary time machine for the human mind. Dostoyevsky discovered the meaning of life in one. Mendeleev invented his periodic table in another. Neil Gaiman dreamt his way to a philosophical parable of identity. We are born dreaming. As we go through life, dream-sleep plays plays a major role in regulating our negative emotions. When we dream, we are our most essential and sovereign selves — our shadows the starkest, our creativity the wildest, and all of it, crucially, ours alone. We build and unravel entire worlds, answering to no one but ourselves — and even that, only hazily. Graham Greene celebrated this sovereignty when he observed in his dream diary that “it can be a comfort sometimes to know that there is a world which is purely one’s own — the experience in that world, of travel, danger, happiness, is shared with no one else.”