focus

A focus is a hearth. In Latin the word focus meant the fireplace — and not as one furnishing among many, but as the center of the house: the point the household turned toward for warmth, for light, for the cooking of food, the place the family gathered and the fire was kept. To say focus in Latin was nearly to say "home." The Romans even used it that way figuratively, the hearth standing for the family around it. Everything in the old sense of the word points inward and gathers: the one warm point in the cold house, the place the day comes back to.

The word became geometrical in a single stroke, and we know whose. In 1604 Johannes Kepler, working on optics and the paths of light through lenses, needed a name for the point where rays converge — the burning point, where a lens gathers sunlight tight enough to scorch — and for the foci of a conic section. He reached for focus, the hearth, because the burning point of a lens is a little hearth: the place the light is gathered until it catches fire. The metaphor is exact. A magnifying glass makes a hearth out of a sunbeam. The word entered English optics within fifty years, through Hobbes and others, and from the lens it spread — the point of convergence of anything, and then, by 1796, the center of activity or attention. To focus the mind is to do with thought what the lens does with light: gather the scattered rays to the one point where they are hot enough to work.

The everyday meaning is younger than it feels, and worth dating, because the abstraction happened in stages you can watch. First the literal hearth; then Kepler's burning point; then the optical fixed point; then, only at the very end of the eighteenth century, the figurative "center of attention." Each step keeps the same shape — a gathering toward one point — and loses a little of the fire. By the time we say "let's focus" in a meeting, the hearth is three metaphors down and entirely invisible. But the structure survives: to focus is to bring the dispersed to a center and hold it there. The word still does, in the mind, what the fireplace did in the house.

The deepest origin is honestly unknown, and the honesty matters because of one spectacular false friend. The Romance words for "fire" — Italian fuoco, Spanish fuego, French feu — really do come from focus, because in late Latin the hearth-word elbowed aside ignis and became the ordinary word for fire. So you would naturally expect English fire to belong here too. It does not. English fire is inherited Germanic, from a different PIE fire-word entirely — *péh₂wr̥, the root that also gives Greek pyr and English pyre. Two fire-roots ran side by side in Indo-European, an animate one and an inanimate one; Latin's hearth-word and English's fire descend from different ones, and only the Romance languages took their fire from the hearth. As for where focus itself ultimately comes from — whether the "burn" root or the "shine" root, which would make it a cousin of phantom and photograph — Etymonline declines to say, and so does this entry. The hearth is sure. What lit it, we do not know.