I’m often struck by how little it takes to set off a chain of associations in the mind. The smallest stimulus will do. So long as it’s shaped and weighted to just the right degree, it can trigger a response whose scale completely dwarfs it. I’m sure there’s an intricate array of biochemical processes in our brains that accounts for the way in which these large-scale concatenations are set off by the tiniest of prompts. But I find it hard to imagine beyond the haziest abstraction the elegant interlocking of cells and the microscopic transactions they broker; the chemical messages passed between them and the firing of nerve synapses as the mind’s currents spark and flow. The invisible complexities that sustain us don’t easily lend themselves to metaphor-making—our fundamental technique for bringing the world to heel and picturing it into sense. As George Steiner puts it in Real Presences (1989), metaphors offer “new mappings of the world;” they lay down paths that we can follow in “our habitation of reality,” making that habitation seem something familiar and intelligible. For all that they explain to us, the unseen processes within are poor sources of imagery for the metaphorical cartography that helps us feel at home.