People often feel the presence of someone when no one is there. This may be a way of embodying the fear of the unknown, the ghost under the bed. It may be a near-palpable memory of an absent person, triggered by an article of clothing, a photograph, a scent, an old recording. And it can at rare times be a feeling of immanence, of being close to spirit or divinity. Regardless of the source, the sense of presence-in-absence reinforces a need - which...